Can't writemime plots

I was expecting to be able to do

```
open(file->writemime(file,"image/png",PyPlot.streamplot(args...)),"test.png","w"))
```

In order to save a PyPlot plot (this works for e.g. Gadfly/Winston). Any chance that could be supported?

Start by examining how PyPlot handles writemime and how Gadfly or Winston support saving plots through the same interface. Reproduce the issue with the shown PyPlot.streamplot example and check whether the command can write image/png to test.png. Done means the example successfully saves a PNG plot.

Written by the indexing model from the issue text.