Simplify markdown extension specification

[bhagany]

Small backwards incompatibility here - the we only used the :extensions key of the options map passed to markdown. This tripped me up when I first started learning Perun, because I didn't understand why we were wrapping the extensions map in another map (I assume something other than :extensions was used at some point in time).

[runnabot]

Deployed perun/md-exts. View on Runnable.
_{From Runnable}