Quality Option Fix

Great plugin you have here! Thanks!

I noticed the same quality issues that were in #1.

These seem to fix the issue :tada: 

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1. Quality settings were always overwritten with the defaults
  - _.extend expects the defaults passed in as the first argument
  - http://underscorejs.org/#extend

2. node-imagemagick only adds the quality option if the format is 'jpg' or 'jpeg'. Currently sending in '.jpg' or '.jpeg'
  - https://github.com/rsms/node-imagemagick/blob/master/imagemagick.js#L393