Add support for partial match in `find_comment`

Created by: nnashok

The current implementation of find_comment requires a full match of the string. It would be great if we can search by partial match where some part is known but the other parts may vary. I'm ok with changing the current API with additional args or a new API. The use-case would be something like:

cron.new(command='echo first', comment='echo: First')
cron.new(command='echo second', comment='echo: Second')
cron.find_comment('echo:', partial_match=True)

This will get me all the jobs which have echo: in their comment.