Scrolls-py

**Beta Release** - `0.x.y` should not be considered stable, and backwards
incompatible changes may be made at any time.

ADDED
- Add `scrolls.InterpreterContext.all_commands` and
`scrolls.InterpreterContext.all_expansions` for programmatic
calls from plugins.
- Add `scrolls.InterpreterContext.init_handlers`, to be called
by the interpreter.
- Add `prelude` argument to `scrolls.Interpreter.repl`, allows running
a script before dropping into interactive mode.
- Add builtin: `use-print-on-unified`, which will print a debug message when an
expansion is used as a command. This is automatically enabled in interactive mode.
- Using `q` or `quit` in interactive mode will quit. The preferred way is still
`stop`, but the other two are so common might as well add them.
- Add `trace_banner`, `trace_str`, and `__str__` to `scrolls.CallContext`, for
printing stack traces.
- Add `get_backtrace` to `scrolls.InterpreterContext`. Prints a call stack
and context summary for the current interpreter context.
- Add `python` and `backtrace` commands, under a new builtin handler
(`scrolls.DebugCommandHandler`).

CHANGED
- Automatically enable unified commands for interactive mode.
- `scrolls.InterpreterError` now produces a stack trace in addition to pointing out
the error in the code.
- Heavily refactored `scrolls.interpreter`, breaking it up into several parts.
- Heavily refactored `scrolls.ast`, breaking it up into several parts.

FIXED
- Fix ineffecient creation of `scrolls.ChoiceCallHandlerContainer` instances
in `scrolls.Interpreter`.
- Fix typo in documentation of `scrolls.InterpreterContext.runtime_commands`.
- Return protocol instead of base class for `scrolls.InterpreterContext.runtime_commands`
and `scrolls.InterpreterContext.runtime_expansions`
- Fix many missing links in documentation.
- Fix some inaccuracies in parsing documentation.

**Beta Release** - `0.x.y` should not be considered stable, and backwards
incompatible changes may be made at any time.

FIXED
- Fixed incomplete comment for `scrolls.unified_config`.
- Fixed `scrolls` module missing `UnifiedCommandSettingHandler` at top level.

**Beta Release** - `0.x.y` should not be considered stable, and backwards
incompatible changes may be made at any time.

ADDED
- Add `!else` and `!elif` builtins.
- Add `scrolls.CallContext.else_signal` for plugin support of `!else`.
- Add `scrolls.InterpreterContext.parent_call_context`.
- Add math builtins:
- `**`: Exponentiation
- `sqrt`: Square root
- `round`, `floor`, `ciel`: Rounding
- Add `vlen` builtin.
- Add a base call context (command, name `__main__`, no arguments) for toplevel code.
- Add `use-unified-commands`, which allows expansions to be used as commands.

CHANGED
- Empty parenthesis are now optional for control calls: `!else() -> !else`

REMOVED
- Remove `scrolls.InterpreterContext.in_call`, because it no longer makes sense
to ask whether the interpreter is in a call or not due to base context.

FIXED
- Fix oversight in `scrolls.InterpreterContext.interpret_ast` leading to
incomplete initialization of context for full AST execution.

**Beta Release** - `0.x.y` should not be considered stable, and backwards
incompatible changes may be made at any time.

ADDED
- Add `scrolls.Tokenizer.set_comments_enable`.
- Add `scrolls.Tokenizer.set_newlines_separate_strings`.

FIXED
- Fix `scrolls.Tokenizer.set_quoted_literals_enable` not disabling
quoted literals when called with `False`.
- Fix KeyError on partially filled optional args for callbase commands.

**Beta Release** - `0.x.y` should not be considered stable, and backwards
incompatible changes may be made at any time.

FIXED
- Fix issue in `scrolls.ext.callbase` causing `GREEDY` options not returning
a `Sequence` if only one argument is parsed.
- Fix `scrolls.ext.callbase` not included in package.

**Beta Release** - This version should not be considered stable, and backwards
incompatible changes may be made at any time.

ADDED
Initial release.