Re-enable matplotlib printing as option for init_printing

Re-enable matplotlib as a possible way to png render latex. This
addresses some problems with Windows where using external rendering is
slow and flashes up batch windows. This option is specified through
use_latex by passing 'matplotlib'.

Additionally, the `use_latex` option can specify a list or tuple of
strings, in addition to a single string, to set the latex rendering that
is enabled. This allows users to set both the png and text latex
rendering methods.