Directional Property Shorthands

**Issue Reference: #140**

Five shorthands for writing directional properties

---
```scss
@include margin(null 10px);
```
```css
margin-right: 10px;
margin-left: 10px;
```
---
```scss
@include padding(null 10px 0);
```
```css
padding-right: 10px;
padding-bottom: 0;
padding-left: 10px;
```
---
```scss
@include border-style(solid null dotted);
```
```css
border-top-style: solid;
border-bottom-style: dotted;
```
---
```scss
@include border-width(10px 10px 10px);
```
```css
border-width: 10px;
```
---
```scss
@include border-color(blue red green yellow);
```
```css
border-color: blue red green yellow;
```