Add support for matching a part of a commit message in capistrano 2 deploy finish task

This pull requests adds the ability to match only a part of the commit message. My use case is being able to extract the ticket number from the message and only include that in the HipChat commit log chat message.

For example:

    set :hipchat_commit_log, true
    set :hipchat_commit_log_format, ":message"
    set :hipchat_commit_log_message_format, '^PROJECT-\d+'

The above would extract "PROJECT-123" from the full message "PROJECT-123 - This is a commit.".