Other themes

I've added in two extra theme colours, the default theme and the twilight theme. Plus also a tests.md file to check that each set of styles is being applied correctly. I made some tweaks to the keywords, the biggest of which is to replace the hr styles with a 'heading' line styles - although they can be used for horizontal lines too - it is just a 'prefixed' keyword where 3 or more `---` or `===` are styled the same as the headings. I haven't made the changes to the bespin file and in theory the default theme should be at the route and the bespin and twilight themes should be in the subdirectories.