Make --plots a flag with no arguments

[tbekolay]

Right now you can do

pytest --plots

or

pytest --plots my_cool_plotdir

to pass a directory in which to make plots. However, it's now also possible to set the plot directory in a config file. And it's quite annoying to do something like

pytest --plots nengo/tests/test_ensemble.py

and end up inadvertently making a nengo/tests/test_ensemble.py directory with a bunch of plots in it. So maybe we should just remove the ability to pass in a plot directory through the command line, and have --plots just be a flag.