Parse for numbers in hotfix and feature.
helpers.rb: Update merge description.
Update man pages.
Merge allow-merge-to-use-pull-number (#97) into master

Allow merge command to take pull request number.

This will allow devs to easily merge branches based off their pull
request number.

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Original issue: The merge command can't take a pull request number.
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I should be able to `feature | hotfix merge -n '# of pull-request'` for convenience.