Add support for the doublestroke package

Add support for the doublestroke package

This commit adds the mathed command \mathds that selects a
font more appropriate than \mathbb for typesetting the
mathematical symbols for the natural (N), whole numbers (Z),
rational numbers (Q), real numbers (R), complex numbers (C),
and some others.

As in the \mathbb case, only capital letters are supported,
but in addition one can also typeset a symbol often used for
representing the indicator function (\mathds{1}) and the
letters a, h, k.

Fixes bug #11887.