Tab drilling

From https://github.com/kien/ctrlp.vim/pull/513:

> This patch adds additional abilities to the tab key to expand directories based on the currently selected entry.
> 1. If the user didn't type anything, tab now expands the directory name of the selected entry. Before it didn't do anything
> 2. If the user typed something, tab now expands the directory one after the other while maintaining any other expansion that happened before

Comment from @kien:

> This looks great. But how does it interact with CtrlP's existing tab-completion? Also, since they are closely resemble, is merging them feasible?

Comment from @axelson:

> @kien how does CtrlP's existing tab-completion work? I don't see it listed in the documentation anywhere.

Comment from @kien:

> @axelson see :h ctrlp-autocompletion. You need to type something first.