Finish implementing Signature Help Provider for Functions

[MaulingMonkey]

Related: PistonDevelopers#287
Branch: https://github.com/MaulingMonkey/VisualRust/tree/sighelp

Signature Help Provider

• Basic scaffolding and registration of required classes
• Refactor racer parsing to be reusable
• Parse doc comments for arguments
• Parse signature for signature portion to highlight
• Commit it all
• Eliminate 'extra' markdown syntax from doc comments?
• Remove parsed arguments sections from signature help since it's displayed per-param anyways?
• Add generic params to arguments list?

Bugs:

• Only match exact identifiers, not subsections
• Match single character identifiers
• Keep quickinfo up until e.g. the final ) is typed
• Transition between current parameters when navigating with arrows
• Transition between current parameters when typing ,
• Filter signatures returned by racer to only match the exact identifier, not longer substrings
  (maybe a reason to parse racer.exe's PREFIX ... messages?)
• Performance issues due to complete lack of cached racer info

Preview;