add command "me" to react if the bot was mentioned in a message

A file with possible bot answers can be specified with option "-me".
If no file was given, a default answer will be added. Messages may
contain the placeholder %USER% which will be replaced by the username
who sends the message.
Merge pull request #6 from ralfth/add-me-cmd

add command "me" to react if the bot was mentioned in a message