Setting baseurl via frontmatter

[lukas-mertens]

Is your feature request related to a problem? Please describe.
People often ask to share slides after the presentation. A reveal.js presentation can be shared easily via putting it on a webhost, but sometimes you don't have a dedicated (sub-)-domain for it, but put it into a subdirectory. Now you have to change the baseurl of reveal.js, otherwise all links to other resources are broken.

Describe the solution you'd like
A property in the frontmatter OR docs for the existing setting (if it already exists)

Describe alternatives you've considered

• Fixing the baseurl after html export by editing all resulting files. Painful and unnecessary + error prone