Adds the #groff method to the GroffParser::Document class, allowing the user to call groff commands with additional flags
Improves some methods in the document class to accept custom flags
Merge pull request #6 from roperzh/issue-5

Allow passing flags, fixes #5
Fixes failing tests in the engine model
Merge pull request #8 from roperzh/issue-7

Fixes failing tests in the engine model
Removes a missing keyword parameter, enabling full compatibility with ruby 1.9.x