adds simple tests to work through adding `locals` to jade rendering

hi @chmanie, thanks for the helpful karma config module!

i was curious if you'd given any thought to supporting rendering `locals` in the jade rendering?

this pr includes some pretty simple code to use the `config.locals` object, if it exists. i also didn't want to assume which testing framework you prefer, so i did the most dead-simple assertions(*) i could think of.

`npm t` from this repo will show that compiling WITHOUT locals works fine, and using locals produces the desired output.

our use-case is simply ensuring that our jade builds work properly in testing, just as in development and production. without the locals, many of our templates fail to build.

let me know what you think! (or if i'm barking up the wrong tree -- very possible -- i didn't see any issues to this, so thought i'd send this your way.)

*p.s.: if you have a preferred way of adding some tests in, happy to update this pr -- the tests i've got here will become unmanageable basically immediately.