Plots don't render at all on Windows

[CAM-Gerlach]

Currently, nothing is rendered at all for plots in docrepr on Windows, even though it appears to work on Linux in an identical environment. This likely has to do with temp file/directory handling or other OS-specific differences. See the discussion on #30 and further discussion on #32.

It would be really nice to have this for v0.2.0, since it is unclear if it was working on Windows before (but seems likely so), but it can be pushed if necessary. Thanks!