Html embed exporter

[juhasch]

• Add an exporter for embedding graphics in HTML nbconvert output
• Update documentation on exporters
• Add link to documentation in README.md

[jcb91]

I'm not sure why codecov seems to have decided to add statuses to PRs again, I thought it was disabled 😦

[jcb91]

but anyway, I think it's complaining because your new exporter isn't tested to the extent of the rest of the codebase.

[jfbercher]

The new exporter embedding linked images is certainly a good idea.

[juhasch]

@jcb91: True, a test is still missing. I will add one before merging.
@jfbercher: It would be nicer to have entrypoints for postprocessors, so one could add a --embed flag or so. But for now I think this is the best option.