add configuration support

[clausreinke]

So far, we've got a fixed configuration built-in (mostly the default config, with target set to ES5). There is also a hack to support loading different lib.d.ts files. In the future, we should try to support configuration in more standard ways, perhaps via startup flags, more likely via the new .tsconfig files.
Related issues: #41 #43.

[clausreinke]

For tsconfig.json, follow microsoft/TypeScript#1667 and microsoft/TypeScript#1692