Initial commit of Gitlab CLI shell. Tab completion for commands.

Using Readline, I wrote a shell-like interface for the Gitlab gem, based on the work done in the CLI.  Tab completion works for commands.  See this Gist for some example output: https://gist.github.com/asedge/719ec984d708e233fb06.  Obviously I made use of the output tables to make my life easier.

I haven't written any tests yet or updated the ones I probably broke by re-arranging some of the CLI code.  I will do that but wanted to get some feedback on whether or not this was a good thing before moving forward.

Some additional features that would be nice for the shell:
1) Legitimate help command. Show which arguments need to be set and with what data.
2) Possibly write/read command history
3) Possible rc file to set endpoint+private_token. (could be shared with the CLI)