bring virtual fs support: decouple readFileSync from parser

[ivankravchenko]

Bring virtual filesystem support to jade!
I've referred usage example in test case.

[puzrin]

+1.

It's very useful for complex projects, splitted to multiple packages. For example, now i have to monkeypatch "include" to make it understand the same search logic as require provides.

[ForbesLindesay]

I like the idea of doing this. Lets make it part of the 2.0.0 release, since adding an option is a (small) breaking change.

[ForbesLindesay]

Thanks for all the work you've put into this. I am going to land this feature in 2.0.0 but will do so slightly differently - by allowing the user to override the "loader" module in compilation.