* Remove unneeded d.ts files from extensions
Moves most extensions to use the lib files for the standard library that typescript provides.
* Remove a few more node.d.ts references
Makes a few changes to the markdown extension:
* Enables explicit null checks.
* Enables no implicit any checks.
* Switch to using TS 2.0 lib files.
This change required removing the mocha declaration from the shared `declare.d.ts`. Everything seems to compile fine without it.
