Provide a command for toggling the status bar

Fixes #24 by providing a `status-bar:toggle` command in https://github.com/atom/status-bar/commit/00fc99029832587ad952ee0792d9a470b5ff7d40, similar to the tree view package's toggle command.

Also, did some minor cleanup in https://github.com/atom/status-bar/commit/5a30dcade918712a78b5965a9ef746f84f4f5bfc, and fixed a strange issue in https://github.com/atom/status-bar/commit/df05afce446e804f9d1f3b49c9636a38fd7ed4af which was blocking me from finishing this PR for a while.

cc @benogle @kevinsawicki