is-svg

Introduces an optional option, to disable XML validation. Disabling the XML validation makes SVG recognition a lot faster, at the cost the SVG XML is not validated. Difference in processing time: Resulting in about ~12× to ~60× times short processing time. ToDo: [x] Needs a unit test covering use cases with validation disabled Code I used to measure performance: Resolves #42 (with the note that the actual degradation in performance as described in that issue, I could not reproduce, yet this is significantly reduce parsing time)