Use matplotlib style sheets to avoid modifying hidden global state

[effigies]

Matplotlib allows configuration to be wrapped in style sheets, which can be used as context-managers (with statements).

It would be good to provide a nipype.utils.style_sheet context that we set with the current defaults. Any internal calls to plotting would then happen within that context. This would give users a defined way to customize nipype styling, as well as avoid infecting their own plotting environment.