Adds configuration for progressive JPEG's

We've got a client system that doesn't effectively render progressive
jpeg images.  As such, we're looking to have a configuration value to
control the `progressive` flag on JPEG images.
Merge pull request #401 from SONIFI/master

Adds configuration for progressive JPEG's
WebM support
Merge pull request #402 from thumbor/webm

WebM support
Bump to 4.8.1
Merge branch 'master' into statsd