Compilate sub-directory support

[ghost]

Currently works many to one - input to output directories, just by passing in another option.

require("dot").process({
    path: ./templates/,
    destination: ./lib/render/,
    subDirectories: true // false by default
});

[epoberezkin]

Not backwards compatible, so can only be added to 2.0.0

[epoberezkin]

@ghost I like the idea of compiling sub directories. It can be added to the current version as it is based on the option. I am not sure whether it's better to implement directory traversal option here or use some external library (like glob). It won't be bundled in if used in browser and it would substantially simplify the code... @olado what do you think?
In any case, it needs the tests - probably it's easier to add them once #211 is merged.