docstrings/sphinx: Support "trim_doctest_flags" option

[thatlittleboy]

This trim_doctest_flags option allows the user to remove doctest flags like # doctest: +FLAG and <BLANKLINE>, present in example snippets within docstrings, from the parsed output. E.g.

def f(x: int) -> int:
    """Test function.

    Example:
        We want to skip the following test.
        >>> 1 + 1 == 3  # doctest: +SKIP
        True
        And then a few more examples here:
        >>> print("a\\n\\nb")
        a
        <BLANKLINE>
        b
        >>> 1 + 1 == 2  # doctest: +SKIP
        >>> print(list(range(1, 100)))    # doctest: +ELLIPSIS
        [1, 2, ..., 98, 99]
    """
    return x

This is default behaviour in Sphinx. See original discussion / feature request on https://github.com/mkdocstrings/mkdocstrings/issues/386.

The idea is to emulate this behaviour for this sphinx parser in griffe.

Pre-requisite / blocker for this issue seems to be #7 .