Add dx and dy attributes for SVG text

dx and dy can be useful to relatively position text. I would like to use these attributes within my SVG axes and as mentioned in the d3 [docs](https://github.com/mbostock/d3/wiki/SVG-Shapes#svg_text):

> additionally, the text can be offset from the anchor using dx and dy attributes. This offset is particularly convenient for controlling the text margin and baseline, as you can use "em" units which are relative to the font size.
...
It's possible that there is a better way to specify the text baseline using SVG's baseline alignment properties, but these don't seem to be widely supported by browsers.

Signed the CLA.