Work-around for full-width plots.

Introduces an unofficial work-around for the behavior introduced in
0.8.0, where we reserve extra space for the first and last tick labels.
 This makes it impossible for plots to span the full width of their
container.

This work-around overloads the reserveSpace option, which normally only
applies when show = false.  Now, if show = true and reserveSpace =
false, we skip the step that allocates extra space for labels.

We’ll introduce a proper solution in 0.9, but this should provide a way
for users to move forward in the meantime.