Add a rem calculation function

# Add a `rem` unit calculator

Status: **Opened for visibility**
Reviewers: @jeffkamo @ry5n 

## Changes
- Added a new settings directory
- Added a setting for your root element font-size value
- Added a strip units function (from Bourbon)
- Added a rem calculator function

## Notes
- To use this function, just set `rem(desired-pixel-value)`. Like so:
```scss
p {
    font-size: rem(14px);
}
```
- If you want to, you can specify an alternate rem-base to use to calculate a value. By default, the rem-base is `100%` which is equal to `16px`. You can do that like this:
```
font-size: rem(14px, 125%);
```