Add px and py rules to padding rules.

A px rule is a padding rule that only affects left and right padding(horizontals hence the x) and is written like so:

```
.px {
  padding-left: auto.
  padding-right auto.
} 
```
A py rule is a padding rule that only affects top and bottom padding(verticals hence the y) and is written like so:
```
.py {
  padding-top: auto.
  padding-bottom auto.
}
```