Add more tab commands

[keeakita]

This adds two new tab commands: tabr/tabrewind/tabfi/tabfirst and tabl/tablast. These take you to the first and last tab, respectively.

Changes Proposed in this Pull Request:

• Add :tabrewind command that takes you to the first tab in the pane
• Add :tablast command that takes you to the last tab in the pane
• Add :tabr, :tabfir, :tabfirst as aliases to :tabrewind (to match vim)
• Add :tabl as an alias to :tablast (to match vim)

I have written tests for:

• New features introduced)

However, I was not able to run tests on my laptop (Fedora), it just popped open a bunch of File Open dialogs and failed random tests. I'm hoping that you have CI on this repo or can get it running on your own :(

[keeakita]

Ah, looks like Travis is similarly broken :|

Let me know if there's any manual testing / screencasts / etc I can add to the PR in place of having the tests actually run :)