All of our custom Markdown extensions have priorities that govern
the order in which the preprocessors will be run. It is more
convenient to have these all in one file so that you can easily
discern the order at first glance.
Thanks to Alya Abbott for reporting the bug that led to this
refactoring!
This commit is first of few commita which aim to change all the
bugdown references to markdown. This commits rename the files,
file path mentions and change the imports.
Variables and other references to bugdown will be renamed in susequent
commits.