Allow print media queries

[Paul Smith]

Breakpoints have the mediatype hard coded to be screen. It is currently not possible to have a valid media query which references a different mediatype.

Removing that hard coding allows a breakpoint to be added like this

'breakpoints': (
  ... // as per existing
  'print': 'print'
)

making it easier to control the print layout.

I have a pull request ready (my first one ever!) for this change if there is any interest.

[Andy Bell]

Let's do it, Paul! I have just defaulted to screen out of laziness so I'm very up for it being more useful.

[Mario Fehr]

@hankchizljaw as this was fixed with #12 this can be closed?