Support "span" in CSS Grid shorthand

[kevinweber]

Support span values in the column and row shorthand syntax.

@lencioni @ljharb @etripier
#173

[kevinweber]

@robinweser What are your thoughts on this?

[robinweser]

Heyo, sorry for not responding that long.
That PR looks good to me! Will try to merge and release by tomorrow.

[matthewharwood]

@robinweser this caused a regression

it('should expand the shorthand column syntax with span as a start', () => {
  const input = { gridColumn: 'span 1' } // error with this valid input
  const output = {
    gridColumn: 'span 1', 
    msGridColumn: 3,
    msGridColumnSpan: 1,
  }
  expect(prefix(input)).toEqual(output) // error
})

Failing branch:
https://github.com/matthewharwood/inline-style-prefixer/blob/master/modules/__tests__/createPrefixer-test.js#L559

    TypeError: Cannot read property 'split' of undefined

      at gridRow (plugins/grid.js:94:3613)
      at Array.grid (plugins/grid.js:105:5)
      at prefixValue (utils/prefixValue.js:10:86)
      at prefix (createPrefixer.js:42:190)
      at Object.<anonymous> (__tests__/createPrefixer-test.js:567:18)

I hijacked this issue:
#176