Add custom-css-link to html-header

[donfranio]

This way you could use reveal-hugo unchanged and do smaller formatting changes in your hugo slides.
Just add

[params]
custom_css = ["custom.css"]

to your configuration and edit custom.css in the folder static/css.

[donfranio]

Can someone please help with this build-Error?

[joshed-io]

@donfranio Thanks for the contribution! I fixed the build error, it was due to an out of date build image on Netlify. Can you add a quick note in the documentation about how to use this? Then I'll be happy to merge it in.