Switching leading_underscore convention to option

Rather than have this be an entire convention, create an option
`allow_leading_underscore` which regardless of convention specified
allows you to prefix names with underscores to denote them as private.

Change-Id: I380cdf84d8e5d263b76b033137a9e4a69025d9ca
Reviewed-on: http://gerrit.causes.com/45099
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>