Implement SVG Importer as a Popcorn Parser, Allow Any Parseable Files to be Dropped into Butter

[banksJeremy]

(Related to t1166.)

After implementing and discussing approaches to adding the Drag-and-Drop import to Butter a couple of times, Bobby and I settled on a nice one. Using Parsers, Popcorn users can already define way of converting files to Popcorn events, based on their file extension. The SVG handling should be implemented as one.

It seems like Parsers haven't been used too much with Butter (getting them working has been confusing); letting any parseable file be dropped directly into the timeline fits our use case perfectly and make it possible for end users to use Parsers without needing to touch HTML (for example, to import a transcript file).

[dseif]

Cool this sounds great! Looking forward to seeing this :)