Make hoedown library optional (#121)

Add a compile-time option with_hoedown to enable hoedown support to
the qmake project files.

qmake CONFIG+=with_hoedown
Fix integration tests (#121)

Only build and run the HoedownMarkdownConverterTest when the
compile-time option with_hoedown was set.
Merge pull request #124 from cloose/feature/hoedown-optional

Make hoedown library optional (#121)