Support zooming out when viewing code

Closes #281
Closes #311
Add markdown processing of source code files

Closes #306
Closes #310
Update markdown extensions
Require '.' when checking markdown extensions
Import LoaderCallbacks directly
Hide progress when exception occurs
Add option to switch back to raw markdown

Markdown files are rendered by default when opened
but can still be toggled back to raw mode which is
saved as a preference.