add submodule discount (https://github.com/Orc/discount.git)
moved sources to subdir app

moved sources for gui application to subdirectory app
removed sundown library

removed subdirectory of sundown library
removed unneeded TextBuffer class
Migrate parser from sundown to discount library
moved discount to v2.1.6
Use mkdio.h and simplify mkd_string call

Replace markdown.h with mkdio.h to be able to call mkd_toc() to generate
a table of contents
Split MarkdownParser in Document and Parser classes

Split MarkdownParser in Document and Parser classes and move the source into subdirectory discount. Add caching of resulting HTML source.
Also remove the slot method styleChanged() from MainWindow.
Merge branch 'feature-discount' into develop