Complete regex highlighting: start, end, skip, content, specials

[lukewilson2002]

The regex highlighting system is not yet complete. I only added the support for single line matches. Multiline matches like multiline comments and strings need to be implemented by handling the End case of the RegexpRegion struct. Also, implement handling the Skip, Content, and Specials fields as well, for added control when matching strings and the like.

For example, matching a multiline string with control sequences (without highlighting the control sequences):

RegexpRegion:
  Start: "\"",
  End: "\"", // match to end of quotation
  Skip: "\\[tvrnb]", // other escape sequences elided
  Content: "[characters][numbers]", // Everything not a string