Vector pretty

This is https://github.com/sympy/sympy/pull/8131 with https://github.com/moorepants/sympy/pull/5. Tests passed on https://github.com/moorepants/sympy/pull/5 and I am +1 to https://github.com/sympy/sympy/pull/8131 (and https://github.com/moorepants/sympy/pull/5 is a trivial fix), so I am going to merge this.