Allow custom builds

**This isn't complete, but here's how I imagine it working once it is:**

Running `grunt` without any options will simple build everything (or at least everything that is ready to be build) just like it is now. *I need to know every "module" that would be included by default currently.*

However, once a `--with` has been specified, all defaults are cleared and the build only contains the modules and dependancies that are required for that build. So if `--with aes,sha256` is specified, a asmcrypto.js file is build containing only aes, sha256 and the corresponding dependancies. In my view, that would include the exports for those functions, which is I need a little clarification.  

Just give my fork a try and see if it behaves how you expect it to.

**Also, heads up:**
Grunt doesn't allow you to specify an option multiple times so something like `--with aes --with sha256` won't work, so instead I did `--with aes,sha256`. If you want some other behavior, let me know.