Allows users to set their own tab completions pattern

[nkakouros]

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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 acdb0588b4933b6c0ad33177cb4cc8b0ed17fe7f on nkakouros-forks:completions-pattern into 292e0803074eb538ab8633a657c53d3a975ba922 on mbland:master.