* Add ability to set custom stylesheets for SVG renderer
This allow to set custom inline CSS and a optional CSP nonce. This
solves the problem mentioned in #103 and is best used with it, as seen
in the added examples. Without this one would have to write a custom
renderer.
* Add note with link to the custom_stylesheets example
* Add ability to set CSS classes instead of inline styles
This allows to set a `ClassName` field in `Style` structs. Setting this
field to anything but "" will cause all other styles to be ignored. The
element will then have a `class=` tag instead with the corresponding name.
Possible reasons to use this:
* Including multiple SVGs on the same webside, using the same styles
* Desire to use strict CSP headers
* Add warning that setting `ClassName` will drop all other inline styles
