Intraword underscores support

[inventionate]

Adds support for Pandoc intraword underscores:

http://pandoc.org/MANUAL.html#emphasis

[Konfekt]

It conceals feas*ible* to feasible, just as when * were at a word boundary.

[alerque]

Thanks for contributing this @inventionate.

In order to get it into a state I could merge upstream I've had to rebase it on master to get rid of the merge conflicts. While I was doing that I also squashed your commits and got rid of the unrelated merge so this should be down to just the relevant bit, including your later touchup. Since this PR was against your fork's master branch my push has affected that. I don't think I've messed anything up but you might take note of that when you pull the next time, you'll definitely want to do a git pull --rebase instead of a merge.

[inventionate]

All right and many thanks!