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.
* a declared module allows to have a sibling module containing experimental API
* this change also cleans up some bogous type pollution that happened because vscode used to be both, a declare module and a global namespace
