Add a function to reverse a linear-gradient()

# Add a reverse gradient function

Status: **Opened for visibility**
Reviewers: @jeffkamo @ry5n 

## Changes
1. Added a function to reverse a `linear-gradient()` function.
1. Added some necessary helpers for strings and lists
1. Moved function helpers into their own directory. These are functions for functions, not ones you'd expect to use in your styles.
1. Added a test.scss file
1. Added some tests for this function
1. Added Grunt to this project to make compiling sass easier

## Notes
- This fixes https://github.com/mobify/vellum/issues/16