Adds filter: Columns

[jdutant]

Adds Columns, a filter for multicolumn support in pandoc's markdown with html and LaTeX outputs.

[tarleb]

Thank you for the PR, I didn't have time yet to look into it more closely. This appears to be a large filter, so the comments in #151 may apply here as well.

I hope to take a more detailed look later this week.

[jdutant]

Thanks Albert, no worries: I am happy to leave that in its separate repo https://github.com/jdutant/columns and post it on the wiki instead (sorry I wasn't aware of the wiki actually). You can close the PR if that's easier.

Also, I plan to modify it by converting LaTeX longtables into xtabs within columns environments (as suggested in this pandoc issue).