Added new methods for horinzontal grids and alignment.

Also removed a strange double assignment that didn't cause any errors/bugs.
Must've been the result of copying and pasting when renaming a variable at some point.

Added new structures to represent the horizontal grid of an artboard.
Started on methods that provide alignment of selected elements.
Added plugins to grow/shrink elements to columns
Fixed bugs where some methods were using column start coords instead of end coords.
Begin to remove all rounding that occurs when converting/manipulating a Horizontal Grid.

Also refactored find nearest column method.
Testing an alternative monochrome palette alogrithm
Plugins for horizontal alignment and resizing.

Updated README to include new plugins, and added section about avoiding crashes.
Merge branch 'horizontal-grids'

Added plugins for working with horizontal grids