Add tag props to grid and cell components

[wab]

As proposed in this issue #41 ...

Which allows to do something like :

<Grid is="ul">
  <Cell is="li" width={3}>
    item 1
   </Cell>
   <Cell is="li" width={3}>
      item 2
    </Cell>
  </Grid>

[azz]

Could you also add docs?

[azz]

Should be a regular dependency

[azz]

The lib doesn't have any docs? :(

[wab]

It's a "sublib" of styled-system ...
https://github.com/jxnblk/styled-system/tree/master/clean-tag

[jacob-fueled]

@wab bump. would quite like the ability to specify the tag to which renders as it current either precludes semantic HTML or forces divitis.