Add support for debugging.

[hcorion]

Visual Studio Code has a fancy built-in GUI debugger. Perhaps it could be possible to add support for debugging nim code. There are multiple articles about it: https://gist.github.com/flaviut/487be89b8abfae2612e8
http://hookrace.net/blog/what-makes-nim-practical/#debugging-nim

I assume it would consist of copying the existing gdb stuff from the C/C++ Extension and then fixing it around for calling the nim compiler as well.

[kosz78]

The current plan is to adopt this extension from Microsoft to allow debug Nim code. Currently this extension allows debug nim code with some weakness and name mangling.

[R3D9477]

Another discussion for this theme #65