Support tab completion in the mini-buffer

[sminez]

The mini-buffer should support optional tab completion configurable based on the mode that it is being used for:

• dmenu style filtering: complete the next whitespace delimited substring
• opening a file: next path segment
• running a command: next matching command
• ...

Making use of this functionality for all applicable cases is probably a bit much for a first step, but getting the functionality in place is a good start.

[sminez]

#25 was raised by @Superpat specifically for adding kakoune style completion to the command mode interface. Doing the same for run mode and supporting completion of programs located in ~/.ad/bin would also be nice to have.