This commit adds playgrounds and a build:dev mode. Now you
can call npm run build:dev to make the linter warn you about
errors without breaking. Also, we now have playgrounds, where
you can use a built version of svg.js to run your own tests.
- added `SVG.HTMLNode` which is the object wrapped around html nodes to put something in them
- moved `defs()` method from `SVG.Parent` to `SVG.Element`
- `SVG()` can be called with css selector, node or svg string, now. Without an argument it creates a new `SVG.Doc()` (#646)
- `add()`, `put()`, `addTo()`, `putIn()` now excepts all arguments accepted by `SVG()`
- `SVG.Nested` is not `overflow:visible` by default
- all `SVG.*` objects now can have a node as parameter when constructing
- `SVG()` does not set a default size anymore
