#76 Allow theme switch (with changes)

[tattali]

Sorry, wrong move I have deleted my initial fork, so I need to create this new pull request to provide my changes. You can close #77 pull request and merge this one, eyes closed.

QLMarkdown.qlgenerator.zip

# switch between themes
defaults write com.fiatdev.QLMarkdown theme old_github
defaults write com.fiatdev.QLMarkdown theme github

# provide github theme
defaults write com.fiatdev.QLMarkdown theme default

# delete preferences
defaults delete com.fiatdev.QLMarkdown theme
# verify deletion or theme keyword value
defaults read com.fiatdev.QLMarkdown theme