Add initial bash completion script

The script can be used to auto-complete commands, image names and
container ids from within a bash prompt.

This partially resolves #336.
Add description and usage information
Missed -entrypoint from 'docker run' options
Clarified bash completion limitations
Still missed -entrypoint from 'docker run'
Merge pull request #1488 from KSid/336-bash-completion

* Contrib: bash completion script