add basic tab completion

Related to #80.

This is __not__ ready to merge, but I added a proof of concept tab completion to Pulldown. Just add `. <(pulldown completion)` to your bash/zsh config file.

@phuu if we could get (and cache) a list of all packages on CDNJS, that would be good if we could offer tab completion on all of those. As long as we cache the list and don't make the request every time.