Add custom markdown syntax for coloring text

<color>(text to color) where color can be a SVG color name,
3 characters hex or 6 characters hex code.

Can be used to send colored message from the composer. Other clients
that follow the matrix spec should be able to display them (e.g. riot,
even if it can't send them)

TODO.md

@@ -92,7 +92,6 @@
 - Way to open context menus without a right mouse button
 - `smartVerticalFlick()` gradual acceleration
 - Make banner buttons look better
-- Way to color HTML from the composer
 
 - Choose a better default easing type for animations
 - Make HListView scrollbars visible