During debugging, I found it useful to hook all task creation in a
central location, and util::task was the perfect place for it.
r? @pcwalton (or maybe someone else, I'm kinda sending you a bunch of
reviews today because I don't know who better to give them to)
Note that using `servo --devtools http://example.org` doesn't work. In
that case either the port must be specified or the option moved to the
end. This is done by getopts and is the same for other such options,
e.g. `--profile`.
