Added -root flag support

A command line flag `-root <rootdir>` can be specified when invoking
maud. Also, the default server root is set to the dirname of the
maud executable, so a `../maud` will set the root dir to `..`, not `.`
added <yt> tags support
separated custom markups, added <spoiler>
adjusted spoiler:hover color
small fix to dates.coffee

In rare cases, diff may be negative; handle the case with an
early return if that happens.
deleted markups.go
realigned master with hamcha
Merge branch 'master' of https://github.com/silverweed/maud

Conflicts:
	static/src/scss/screen.scss