Bash completion

[carolynvs]

The cli package provides bash completion but isn't hooked up at all. I'd like to do something like rack init which configures it for you.

I'm not sure if going through the wrapper scripts will cause us grief or not though...

[rgbkrk]

We could bring back the old bash completion. It was lovely.

[rgbkrk]

Codegangsta cli's bash completion won't give us some of the advanced stuff we did for tab completion on dvm use for instance:

$ dvm use 1.[TAB]
1.8.2 1.6.1
$ dvm use 1.8[TAB]
$ dvm use 1.8.2

[carolynvs]

May want to hold off of this and #38 until we switch to kingpin. I am hoping it will provide much better completion functionality.

[rgbkrk]

This was brought in via #63.