replace --pylab flag with --matplotlib in usage

Since we want to encourage users to start switching to this mode of
working, we might as well advertise it (and *not* advertise the old
namespace polluting --pylab)
document the --matplotlib flag
backticks don't need to be escaped
Merge pull request #3925 from ivanov/more-pylab-cleanup

Replace --pylab flag with --matplotlib in usage

also some other cleanup in what's new