Add support for profile/configuration generation to transgression

Currently, mozregression generates a Firefox-specific profile before running the regression script. We should add support to do this via inheritance. A script could inherit from transgression, provide a function to generate a clean profile, and then run transgression generic methods.

This will not be included in the first release.

No files, tests, or entry points are named. Start by locating transgression's current regression-running methods and determine how a subclass-provided clean-profile generator would fit the release scope. Done means a script can inherit from transgression, generate a clean profile, and run the generic regression methods.

Written by the indexing model from the issue text.