Improve Performance by Adding LRU for ASTs and Fix Issue with Multiple Lines Between Heading and Tag

[pjkaufman]

Fixes #501
Relates to #488

Changes Made:

• Added an LRU cache for AST parsing where the text content of the file is hashed to determine if we already have parsed the file before recently
• The LRU allows for faster rule running since it can allow us to not need to parse files again if nothing has changed and the text is the same (i.e. not a new ignore pattern)
• Updated the heading blank lines rule to not ignore tags as that is not needed and was causing the trailing new line character to get ignored with it causing a bug