This PR makes the context behavior more sane -- now, nesting is allowed, and if you want rc_configurator to return None for a setting when it has not been changed inside the latest context block, you have to use the special rc_configurator functions get, category, and fill with context=True. The old "feature" where __getitem__ can return None would have been baffling for most users.

It also adds a _get_space function so that default gridspec spacing is determined from the axpad, panelpad, and pad settings, and the current font size, tick length, and padding settings, which let me eliminate these clunky subplots.ylabspace, subplots.xlabspace, etc. settings. It also adds [x|y][gridcolor|linewidth] keyword arguments to format, which is something that should never have been included in #50.