Runcommands

- Convert `cd` arg passed to `local` command to an absolute path via the
`abs_path` utility function. This is mainly so that asset paths like
`'package.module:file'` will be handled correctly.
- Normalize default arg names read from config file. E.g., if a command has
an arg named `dir_` with a trailing slash to avoid clashing with the builtin
`dir` function, allow the arg to specified as `dir` in the config file.

- Reworked new config system introduced in 1.0a28. Removed `defaults`. Added
`envs` and support environment-specific globals.

- Fixed handling of `**kwargs`. When collecting the args/options for a command,
`kwargs` is skipped instead of inadvertantly being added to the command's
options.

- Added `keep_slash` flag to path utility functions, because sometimes slashes
need to be preserved (e.g., with `rsync`).
- Fixed `sync` command so that it preserves trailing slashes on the `source`
and `destination` paths its passed.
- Tweaked version regex in `release` task (don't require dev marker).
- Fixed some `local` git commands in release task.

Big rewrite. See the git log for tag 1.0a28 to see all the details. Here's
a summary:

- Focus is more on console scripts and collections of commands rather than
being a Farbric-esque deployment tool, although simplified versions of the
`local` and `remote` commands are still available for simple deployment
scenarios.
- Config files are YAML now instead of custom INI/JSON format.
- Global config is no longer passed to commands as first positional arg.

- Fixed reading of stdin by unbuffering it. The approach used here is derived
from Fabric.
- Added `copy_file` command.
- Added `get_all_list` utility function.