BF: add back matplotlib only directives
BF: add back matplotlib only directives

Apparently needed for ``htmlonly`` and ``latexonly`` commands inserted
by plot directives.

arokem/nipy,alexis-roche/nireg,arokem/nipy,nipy/nipy-labs,alexis-roche/nipy,nipy/nipy-labs,alexis-roche/register,alexis-roche/nireg,nipy/nireg,arokem/nipy,alexis-roche/niseg,alexis-roche/register,bthirion/nipy,alexis-roche/register,bthirion/nipy,arokem/nipy,alexis-roche/nipy,nipy/nireg,alexis-roche/niseg,bthirion/nipy,bthirion/nipy,alexis-roche/nipy,alexis-roche/nipy