rework color management: introduce the trailing props now, instead of prefix and line style, I'm adding the trailing style too. This changed the configuration syntax too.

first scratch for colour support!

kill empty line

add enable-colors (variable) currently unused, will help later when adding colour support

kill ui_notify and export {v,}message

move all cmd_* functions in their own file this detangles definitely the commands from the ui. Various variables and struct from ui.c needed to be exported, aside from that, no functional changes.

optimize redraw_window don't redraw the page if no scrolling happened. There's still need to ``force'' the redraw sometimes, like after a resize or after toggling olivetti-mode, so add a buffer->force_redraw flag for it.

added olivetti-mode!

added about:about, about:blank and about:help pages

add new-tab-url config to specify the default url for new tabs

add fill-column config knob

start to implement `set' in config

config: implement "prefix/cont"

move all styling to defaults.c i.e. don't hardcode text attributes

link the parser in the build and: * add some flags * document them * move the customizable stuff to defaults.c