Rule proposal: unitless px values

[lencioni]

Bad:

{
  paddingLeft: '1px',
}

Good:

{
  paddingLeft: 1,
}

There are some properties that may require the px unit, e.g. lineHeight.

This is similar to #11.

[ljharb]

Similarly, it'd be nice to never allow 0 to have a unit attached, since a unit of zero value is meaningless.