Add Guide Hierarchy Capability

[brodieG]

Currently guidesStr only returns top level elements. If we modified the guides code to return not just the guide index, but their priority, then it would be straightforward to return every guide line with a priority higher than the current line. This would be particularly useful for deeply nested diffStr.

This relates to issue #141 as right now diffStr is not a great alternative as a work-around because it looses all the non-top-level lines that are not part of the context.

[brodieG]

Loosely related to #33