Handle Alt/Esc both
Modify comments so I know what's going on
Protect mod with a mutex

The reason I'm not using defer mutex.Unlock() here is because I don't
want to keep the mutex around while handleKeyEvent() is being
processed.
change delay to 200
Don't handle ModAlt if no M-XXX mappings
Merge pull request #91 from peco/handle-esc-alt

Handle Alt/Esc both

fixes #89
closes #93