Allow custom CSS formatting in published HTML

Perhaps there could be several levels of formatting:

- defaults (`doorstop.css`)
- project settings?
- document settings
- temporary user settings?

For now, all CSS will still be embedded in the HTML. Later, the default CSS could be copied to a `DOCUMENT/assests` directory whenever `doorstop` is run. This is related to #29.

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TODO:

- [ ] add support for default project CSS: `<root>/assets/style.css`
- [ ] add support for default document CSS: `<document>/assets/style.css`
- [ ] add support for custom document CSS: `$ doorstop publish ... --css=path/to/awesome.css`
would probably be a good idea. The initial thought was to write a more generic _wrap_in_html_tag() function, but i didn't want to end up writing an html generation engine...