An ability to append custom scripts and styles

Hi!

I've added options `ext-scripts` and `ext-styles`: arrays of paths for externals styles and scripts. Contents of the files append to the corresponding sections in templates.

Check out this way.

TODO:

- [ ] CLI: parsing that params values
- [ ] add description for params to md and --help info
I'm not really happy with loading these files synchronously. Could (A few lines below there are two more `*Sync` calls I wrote that I don't like as well.)

Could you maybe use promises here? Maybe rewrite the `['scripts', 'styles'].forEach` part as a separate function that resolves with the externals or rejects with a "couldn't read" error. The exported generator function returns a promise anyway, so I think it's okay to wrap the streaming part (lines 61-82) in a `.then` block.