Support a prefix for gaps instead of only __gap

[roshanr95]

#564 was merged recently which looks for __gap variables and does special handling for them. It would be very useful to match a __gap_* prefix as well. This allows the use of multiple gaps within a single contract as well as give them more meaningful names like __gap_purpose. As an example, I like to divide my longer contracts into somewhat independent "sections" based on purpose and give each section its own gap so it can evolve independently as needed.