Initial version of support for multiple markdown converter
Switch hoedown submodule to my fork
Add license header
Moved Options class to app static library
Add support for converter options
Fix CI build
Add general converter options to MarkdownConverter

Add an enum of general converter options to MarkdownConverter that
reflects the current extension options.

Move class Options back from app-static to app.
Remove old discount adapter code
Fix CI build
Fix CI build
Fix CI build
Fix CI build
New integration tests

Add new integration tests for DiscountMarkdownConverter and
HoedownMarkdownConverter classes.
Fix CI build
Add general tab to OptionsDialog
Change discount library name to fix CI build
Implement Markdown converter option

It's now possible to switch the markdown converter library in the
OptionsDialog. The converter is then immediatly replaced in the
HtmlPreviewGenerator.
Merge branch 'feature/hoedown-support-#87' into develop