Xdoctest

Added
* Can now specify zero-args as the command to the xdoctest CLI to run all zero-args functions in a file.
* Add known issue: note about possible want-reporting bug.
* More docstrings
* Add `--version` option to CLI interface

Changed
* You no longer need a comment to denote that a `...` is a continuation and not
a ellipsis. (i.e. you don't need to write `... `)
* Want statements will check against return values in nested continuations
* Cleaned up internal code, private APIs may break.
* Failed doctests will now print their original line prefixes (either `>>> ` or `... ` when possible)

Fixed
* `run_tests.py` now returns the correct error code. (fixes CircleCI)
* Fixed outdated docs in the directive file

Changed
* Improved backwards compatibility. Explicit continuations now work more similarly to the original doctest.
* You no longer need a comment to denote that a `...` is a continuation and not a ellipsis.
* Want statements will check against return values in nested continuations

Fixed
* Removed debug print

Added
* Add skip count to the native runner

Changed
* Renamed several functions in various classes to be private. Its unlikely anyone was externally using them. The change functions include:

* `DoctestExample`: `pre_run` -> `_pre_run`
* `DoctestExample`: `post_run` -> `_post_run`
* `Directive`: `unpack_args` -> `_unpack_args`
* `Directive`: `state_item` -> `effect`

* Modified behavior of `RuntimeState.update` to use the directive effect.

* Added explicit REQUIRES runtime-state, which maintains a set of unmet
conditions. When non-empty it behaves like SKIP.

Fixed
* The REQUIRES directive no longer clobbers the previous SKIP state.

Fixed
* The native runner now exits with a non-zero error code on failure

Changed
* Slight modifications to file structure
* Inherit `util_import` from `ubelt`

Fixed
* Fixed issue with nested functions and exec in older python versions
* Fixed issue in modsplit with multidot suffixes.

Fixed

* Minor fixes to readme and docs