Allows users to set their own tab completions pattern

[nkakouros]

• I have reviewed the contributor guidelines.
• I have reviewed the code of conduct.
• Per GitHub's Terms of Service, I am aware that I license my
  contribution under the same terms as this project's license, and
  that I have the right to license my contribution under those terms.

cc: @mbland

I am using docopts to have argument handling based on the help text of my scripts. This allows me to not effectively duplicate myself all over the place. I have also setup tab completion generations for all my scripts. It is redundant for me to write # Tab completions somewhere in each script.

This PR allows the user to define their own tab completions pattern. This way they can place the hint in the script help (eg have a line that reads # This command supports tab completion), set it to '' which means that tab completions are enabled for all scripts, etc.

[coveralls]

Coverage increased (+0.006%) to 95.003% when pulling acdb058 on nkakouros-forks:completions-pattern into 292e080 on mbland:master.