Bump comment

Adds the ability to prompt the user for a comment about their change (the reason for the bump).  This comment is appended to CHANGELOG.md in the following format:
```

<new_version>
--------
<user> - <comment>
```

Also updated Git plugin to 'git add CHANGELOG.md' during ":after_bump"

Future ideas:
- Pass comment to a git commit.
- Pass comment to various communications plugins (we don't currently use any of these, so may need help here).
- Pull windows username as well as UNIX "ENV['USER']"