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)

@minrk, hope you don't mind, I also included a minor backticks fix in the
what's new document (I was seeing the backslashes that you had escaped). If
your version of sphinx needs it, though, and the generated documents are fine
with the extra backslashes and don't work without them, I can remove that last
commit.