Add support for Emscripten platform/toolchain

Toolchain is Clang-compatible and so is mostly trivial. Resulting applications can be run using node.js, which we already support.

One small issue is that the Emscripten compiler warns when using absolute include paths. There should be an option in the base cpp module to convert include paths which are subpaths of the project.sourceDirectory, to relative paths. There's a command line option to disable these warnings but obviously it's desirable.