Add support for tab completion in bash

Summary: when the bash_completion script is loaded, the user
can use the tab key to automatically complete the name of the
buck command, or if there are multiple commands beginning with
the same letter(s) suggest commands to use.

Test:

Install the completion script:

  $ ./scripts/bash_completion

Suggestion of all available commands:

  $ buck <TAB>
  audit build cache clean install project targets test uninstall

Suggestion of available commands with given starting letter:

  $ buck c<TAB>
  cache clean

Completion of command with given starting letter

  $ buck b<TAB>

("b" is auto-completed to "build")

Change-Id: I573808900811ed79733895f34cccb388535c80b0