Support multiple screens

[ghost]

When the active tag on more than one monitor is set to treesome, treesome handles windows very irregularly.

This might be the same issue as… https://github.com/RobSis/treesome/issues/8 …but the behavior doesn't seem to be width and height being swapped. I'm not sure what's happening, actually.

Here's a video: http://a.pomf.se/dccdzs.webm All I'm doing in this video is jumping between the two screens and spawning terminals. I'm not using h/v-split explicitly.