Libtmux

- Raise exception for invalid session names. tmux does not
allow names that are empty, contain periods or colons.
- Remove unused `target_session` param in
`Server.attach_session` and `Server.switch_client`.

- 12 - fix logger message when tmux doesn't exist in `PATH`

- 8 new exception `UnknownOption`
- 8 return `None` for options that are valid tmux options,
but unset at that scope.
- 6 major documentation overhaul

- update `which()` to find tmux via `os.environ['PATH']`.
<https://redd.it/4laeut>

- attributes for formatters are now accessible via
`Session`, `Window` and `Pane` objects. `session.name`
is equivalent to `session.get('session_name')`, you can do the
same with other properties in `_info`. `window.name`,
`pane.current_path`, `session.id`, `window.id`, `pane.id`,
`session.index`, `window.index`, `pane.index`, etc.
- `attached_sessions`, `attached_window` and
`attached_pane` are now properties.
- `_TMUX` metadata object changed to `_info`.
- `.findWhere()` is now `find_where`.
- README and usage fixes

- switch to pytest