Use stop event, not finish, for consistency.
Support `auto` for modules that export factories

Most of @mojodna's tilelive modules export factories rather than
providers directly in order to reduce the proliferation of dependencies
on (and instances of, though that's been mitigated with the global
registry) tilelive.
Merge pull request #75 from mojodna/auto-factory

Support `auto` for modules that export factories
Merge branch 'master' into progress-stream

Conflicts:
	test/stream-pyramid.test.js
	test/stream-scanline.test.js