Fix failing size() calls & exclude dataURIs from xlink:href detection

Hi Michael,

I encountered two problems with SVG-Cleaner, both somehow related to each other:

1. When a cheerio `find()` call fails for some reason (see 2.), it returns an empty array `[]`, which doesn't have a `size()` method and thus results in an error (the process exits). As the `size()` method does nothing else than returning the `length` property (which is a native Array property as well as you know), I simply changed it to this. Not really nice, I know, but effective. I'd leave it up to you how you'd like to fix this issue though ...

2. I happened to encounter an SVG file containing a data URI as the value of an `xlink:href` attribute (don't really know if this valid at all, but this is what the SVG was like). Your `findReferencedElements()` method would accept this as a valid ID reference (which it isn't of course), leading to an error (being the reason for 1.). This pull request includes a patch that prevents those data URIs from being registered as ID references.

Btw. do you intend to work on SVG-Cleaner any longer? Port the missing parts of Scour? Scour itself *might* be improved in the future, as there's [a new maintainer](https://github.com/oberstet/scour) (@oberstet) now ...

Cheers,
Joschi