Markdown 2.5 needs mdx_ for the asciimathml extension.
As such, we can't filter out mdx_ prefix from markdown extensions.
Make configs in makeExtension an empty dictionary, not None
Flip the if block contents of the dialog box branch checking for the lock
so it doesn't confuse the user
Since the Mikibook.settings also has version indicators for
the changelog to work properly, we'll also need to check
the version here too to make sure the version string in Mikibook.settings
is also updated appropriately.
Fix #40 by force-setting a dummy version string after an attempted
nbListMigration