Unit fallback for properties with multiple values.

Two mixins that allow for fallbacks of properties that allow for multiple values. rem and px fallbacks are both provided, and both allow for single unit fallbacks. Base/reference size is `16px` by default but can be changed on a per-usage basis.

### Arguments
`calc-rem($property, $values, $base);`
`calc-px($property, $values, $base);`

`$property` CSS property to create fallback for.
`$values` List of values to be calculated.
`$base` Base/reference size (default: 16px)

### Usage
```sass
.some-box {
    @include calc-rem('font-size', 32px 16px);
    @include calc-rem('margin', 20px 10px 20px 0, 12px);
}
```
__Output__
```css
.some-box {
    font-size:  32px 16px;
    font-size:  2rem 1rem;
    margin:  20px 10px 20px 0;
    margin:  1.66667rem 0.83333rem 1.66667rem 0rem;
}
```

#### Other
Pull request also contains a function for striping units as it is needed for the above mixins, and a variable typo correction.
__strip-units() Usage__
```sass
strip-units(32px);
```
__Output__
```
32
```