Consider providing directive for ignoring warnings

[astrofrog]

In #84 I added a new option +IGNORE_WARNINGS to ignore all warnings from a given line in a doctest. As @bsipocz mentioned in that PR, it might be worth also having a directive that could apply to code blocks, e.g:

.. doctest-ignore-warnings::

    >>> ...

Note that we don't need a directive for the whole file since I think that can be done with:

.. testsetup::

    >>> import warnings
    >>> warnings.simplefilter('ignore')

Before we work on a directive though, I wasn't sure how we should deal with the case where multiple directives apply, e.g. doctest-requires and doctest-ignore-warnings. Is there a preferred syntax for this?