Add unsharp mask (anthonynsimon#34)

* Add unsharp mask func

* Add unsharp mask amount

* Add unsharp mask doc

* Add unsharp mask tests

* Add unsharp mask to README

Reduce loop vars

Optimize spatial filters (anthonynsimon#24)

* Add bechmarks

* Add Dilate and Erode tests

* Improve performance of sort

* Use edge extension instead of conditions in loop

Optimize noise (anthonynsimon#23)

* Add Benchmarks

* Reduce benchmark image size

* Simplify implementation

Extract neighbors (anthonynsimon#18)

* Extract get neighbors operation

* Add Dilate and Erode functions

* Refactor and add doc

* Switch to radius based spatial filters

* Add dilate and erode to README

Add package comment

Fix typo

Remove duplicate functionality