updating txt2tags markup for making it compatible with geany-themes

I've followed your recommendations for creating a branch and making the PR from this one.
I've updated the marks, adding a markup_ prefix before them.
I've totally left alone the markdown and rst syntax, leaving this to people using them, so they can either follow the new marks or keep the older ones if it's enough for them. The new markup could also be used with many other syntax: wikipedia, AsciiDoc, bbcode, creole etc

I'll PR some colorschemes soon