Add ability to filter image before text extraction.

[hkolbeck]

Fabric.js ships with a bunch of filter operations that could be super useful, and are easy to apply. The UI for these needs to be roughly the same to #14

Application to BG image: https://stackoverflow.com/questions/20119527/how-to-apply-filter-to-canvas-backgroundimage-in-fabric-js

Filter demo: http://fabricjs.com/image-filters
These filters seem promising:

• Invert
• Black and white
• Remove color
• Brightness
• Contrast
• Sharpen