Allow turning on screenshots for failures or selenium errors individuall...

...y

based on https://github.com/beatfactor/nightwatch/pull/150/files
@beatfactor Can you recommend a way to do this screenshot and terminate without the test continuing (pause maybe)?
With this current way of doing it, the test will continue to run as the saveScreenshot is async, and we don't want to terminate yet until the saveScreenshot is actually saved.