Refactor in BashateRun class

Grab the module level functions that seemed to belong
under the 'runtime logic' category and tuck them into
a BashateRun class that should encapsulate the options
and checks that are used in doing a bashate run against
a set of files. Primarily, this makes it easier to unit test
the bashate logic, but also opens up some API possibilities.

Leaving the 'check_*' functions at module level, as they have
a relatively simple contract they need to fulfill for the time
being.

Change-Id: I4e2a30a82501b6ec29d06b49ccba63418cc39fa0