Additional CSS Should create directories

[Havvy]

This seems to be a regression from 0.0.28, but when I put in the config that additional-css = ["theme/reference.css"], it complains:

[havvy@ref:~/workspace/rust/reference]$ ~/.cargo/bin/mdbook build
2018-01-25 20:21:28 [INFO] (mdbook::book): Book building has started
2018-01-25 20:21:28 [INFO] (mdbook::book): Running the html backend
2018-01-25 20:21:28 [ERROR] (mdbook::utils): Error: Rendering failed
2018-01-25 20:21:28 [ERROR] (mdbook::utils):    Caused By: Unable to copy across additional CSS and JS
2018-01-25 20:21:28 [ERROR] (mdbook::utils):    Caused By: Unable to copy theme/reference.css to /home/havvy/workspace/rust/reference/book/theme/reference.css
2018-01-25 20:21:28 [ERROR] (mdbook::utils):    Caused By: No such file or directory (os error 2)

I would expect mdbook to actually make the theme directory specified and then put reference.css into it.

[steveklabnik]

@Michael-F-Bryan is there a way we could get a point release with this change in it?

[Michael-F-Bryan]

Sure thing, I'll make another release when #590 is merged.

[Michael-F-Bryan]

@steveklabnik done!

[steveklabnik]

Thank you! ❤️