Add cleanup to prune old branches.

new file:   cleanup
Add prune option to feature.

Will work on adding documentation for prune.

deleted:    bin/cleanup
modified:   bin/feature
Don't delete or preview current branch.

Add whitespace for clarity.
Add docs.
Modify prune description. Return result for safe_run.

modified:   bash_completion.sh
modified:   lib/git.rb
modified:   man/feature-prune.1.ronn
Merge branch 'master' into cleanup
Merged master into cleanup and removed cleanup.

deleted:    bin/cleanup
Check args before any pruning.
Forgot to exit script with invalid options.
Modify argument count.
Revert "Forgot to exit script with invalid options."

This reverts commit 89d70db1497e7509e17ee1ebd2602037040feca5.
Revert back to desired changes.

Accidentally reverted to wrong commit. This should undo those changes.
Prevent accidental pruning with confirm().
Change options to preview and clean.

Fix piping issue in delete().
Change option flags in helpers to mandatory.
Make number of arguments required dynamic in helpers.

Add documentation to feature.1.ronn.
Change bash_competion back to original mode.
Use unless instead of if(!expression).
Modify option descriptions.
Generate man documentation.

Remove argument checking.
Add argument checks back.
Use branches_cmd to substitute commands used several times.

Use abort() to write to `stderr` instead of `stdout` and automatically
exit the script.
Run commands instead of piping.