Add support for size in pixels when available

[meh]

Closes #6.

Note that this is a breaking change, since it adds public fields to the struct, so it should probably mean a minor version bump.

Also uses winsize from libc since it's available.

[coveralls]

Coverage increased (+1.4%) to 93.75% when pulling 1577599 on meh:master into b9a2660 on softprops:master.

[softprops]

Sweet thanks @meh. I'll get a new version published soon