Add extra stylesheets using css_files in config

[peteruithoven]

See: #69
(Only tested with .documentup.js file)
Also ran tests, the first Markdown test failed, but that's probably not related.

[peteruithoven]

Any feedback would be appreciated

[jeromegn]

Did you add a test for this? I know it's not the bet test setup on this project. This thing is old!

I'm not sure this would work out of the box, but maybe it does. I don't have the whole project's "flow" inside my head anymore :(

Edit: Feel free to fix the broken test.

[peteruithoven]

I'll admit I didn't write a test, but I did test it.
I tested it by running DocumentUp locally and opening a test repository where I added stylesheets to the document up config file:
https://github.com/peteruithoven/DocumentUpTest
https://github.com/peteruithoven/DocumentUpTest/blob/master/.documentup.json
Because the config's from the file can so directly be used in the template there where not many changes for mistakes.

But I'll have a look at the tests

[peteruithoven]

Fixing the current tests seems quite complicated: #86

[peteruithoven]

I'm not sure how to add tests for this, I don't see any tests that check the outputted html or the template system in general.
Would it be an option to merge anyway?
(Not that this is your issue, but otherwise I need to host my fork temporarily somewhere else.)

[jeromegn]

Sorry about that. I have to close this. There's been a complete rewrite of the app.

Adding the functionality should be easier now.

However, I'm thinking it might be easier to specify a theme, and submit the theme as a PR.