Cmd-line Extension

There's a start for this [here](https://github.com/ChrisPenner/rasa/tree/master/rasa-ext-cmd).

The idea here is to have a parallel to Vim's `:` cmd bar or emacs' `M-x`;

Rather than each extension needing to figure out its own way for users to be able to run custom
commands, it's better to have one central extension that they can all tie into. Currently `rasa-ext-cmd` has two utilities:
- `addCmd` : Register a command so it can be found by users in search, or executed by entering the command's alias.
- `runCmd` : Run a registered command by name, in most cases extensions can just run the action representing a command directly, but this may come in handy for extensions that wish to implement some sort of command searching utility.

Work to be done:
- Figure out a good way to let the renderer ([Slate](https://github.com/ChrisPenner/rasa/tree/master/rasa-ext-slate)) render and accept input as a cmd bar. This may involve codifying some sort of 'minibuffer' idea.
- Figure out some sort of tab-complete/fuzzy-find interface. It would be nice if this were general enough to be shared across all extensions that need similar functionality. A solution for this may already exist.

This issue is for discussion about the idea and possible implementations or research into possibilities.