Add embed SVG directive

[gastmaier]

Instead of embedding SVG on the page using the image or figure directive, allow to embed the SVG source code on the page itself, allowing the vector and rest of the page (parent) in a single DOM sandbox.
This allows to create hyperlinks on the SVG linking to the page, among other interactions, such as hover effects.

they syntax shall be:

.. svg::

[gastmaier]

Candidate at commit 484dfdd