Accept existing Vignette URLs

Accept existing Vignette URLs for getThumbURL(), in order to change the
mode of an existing thumbnail or convert a full-sized image to a
thumbnail.
Modify test to include existing thumbnail mode
Code review changes
Bump version for latest changes
Bump version
Merge master branch into issue-12
Merge pull request #16 from Wikia/issue-12

Accept existing Vignette URLs
Merge remote-tracking branch 'origin/master' into issue-17

Conflicts:
	dist/vignette.amd.js
	dist/vignette.cjs.js
	dist/vignette.js
	src/vignette.ts