Collect "from imports" as referencable names for docstrings

[lagru]

This makes it possible to use symbols made available via "from imports" within the same module. This lessens the need to declare types manually.

E.g.

from calendar import January as Jan, Day

def last_day(m):
    """
    Parameters
    ----------
    m : Jan

    Returns
    -------
    day : Day
    """

will work out of the box without having to declare Jan or Day in config. This also works for relative imports.

This feature is related to #20.