Feature/inner content scoped slot

[wrabit]

Hi!

Really like your plugin. Using it in a project at the moment and have had to adapt it for IE11 support (no ForeignObject, so I am using <text> in the svg). What I've done:

1. Made the ForeignObject / default slot render only if default slot present (no breaking changes)
2. Added a “inner-content” slot so we can use anything inside the SVG, in my case <text />
3. New named slot has access to the radius value
4. Updated the demo to show this

Would you consider a merge? 😶

p.s. ForeignObject crashes the whole page when used in IE11 (haven't tested the full scope of other browsers)

[wrabit]

Reference to #7