Add support for variables

Currently `fill` and `stroke` attributes can be changed by this plugin, which is very useful and great.

In a particular case I have a SVG that is used as a `mask` `image` and it uses a `radius` variable that I want to change when inlining that SVG. This plugin would be even more useful if it could accept a list/array of variables and their values and inject these into the to be inlined SVG.