support destSVG in toSVG params

[qrohlf]

Resolves #134

Implement the destSVG functionality to match the documented option that was previously broken. Anyone who wants to use the destSVG functionality before this PR is merged/released can npm install trianglify@next to get the contents of this branch :)

• [x] Implement missing functionality
• [x] write tests
• [x] add an example to the examples folder
• [x] review documentation for accuracy
• [x] cut a point release