This fixes issue #733. Now if the user focuses output right but is at
the rightmost monitor, the focus will wrap the the leftmost monitor.
This commit adds a new function, swayc_opposite_output, which selects
the opposite output given a position and a direction. Now, when calling
output_by_name, we first check if there is an adjacent output to switch
to. If that fails, we call swayc_opposite_output to handle wrapping.