added option for custom extensions

currently the list of Markdown extensions is hard-coded:
https://github.com/tiddlyweb/tiddlywebplugins.markdown/blob/a3d2df60db005d8ce79eee33235516dd07eeb5b3/tiddlywebplugins/markdown/render.py#L17

this means that TiddlyWeb instances cannot choose to include additional extensions (e.g. [checklists](https://github.com/FND/markdown-checklist))

with this change, a list of additional extensions (plus configuration settings) is read from the instance configuration

the test is a bit silly and doesn't cover configuration settings, but better than nothing