emscripten support

[Jonathan Dunlap]

Is it possible that emscripten 
(http://en.wikipedia.org/wiki/Emscripten) could be utilized with 
LDC? I've been interested in emscripten as it allows taking C++ 
code and compile it to JavaScript, with asm.js features. ASM.JS 
is a pain to write by hand, but it's great to use when a compiler 
targets it for you... resulting in 5-10x performance improvements 
in Firefox and around 3x in Chrome.

Rust is looking at using emscripten too:
https://github.com/Yoric/Mozilla-Student-Projects/issues/33