Second alpha version.
- Attempt to fix buffering issues when running subprocesses via `LocalRunner`.
I say "attempt" because this is pretty complex and hard to get just right.
- Allow arbitrary config options to passed via the command line via `-o`; these
options take precedence over config set via config file.
- Improve (color) printer utility. Put the various color printing functions in
a class and create a default instance of that class so that this instance can
be imported instead of having to import the functions individually.
- Print warning, error, and debug messages to stderr by default.
- Make it easier to determine if stdout and/or stderr should be hidden by
adding some utility class methods to the `Hide` enum.
- Only set the `hide` option for a command from the global config value if the
command's default is `hide=None`. TODO: Something similar for `echo`, but
that's a bit harder.
- Remove unused imports, clean other lint, add `__all__` lists where
appropriate, etc.