Added Theme support.

[cnak]

Added options to select different themes.

• Themes are now in located under /themes directory
• Added an options panel to select different themes
• Renamed the default theme "markdown.css" to "originalTheme.css"
• Added an additional theme called "cleanTheme.css" as an alternative theme to the original theme

Pictures attached

Thank you

Ced

Options panel

Original Theme alongside the new "Clean" theme

[borismus]

Thanks, looks great!