Add way to include custom JavaScript and stylesheets

[jronallo]

I've done this in a branch here:
https://github.com/jronallo/deck.rb/tree/custom_assets

Could you make a change like this or would you accept a pull request? What would need to add/change in the linked repo for you to accept a pull request?

[alexch]

Sorry for the delay... I like your patch a lot! Before accepting it I'd want some unit tests.

Also you seem to have disabled all the standard extensions, which I find essential. If you really want the default to be extension-free, we'd need some easy API to re-enable them all.