Default style changes and miscellaneous plotting improvements - #166

This PR tweaks the default style a tiny bit to make things closer to matplotlib (e.g. default line width back to 0.8 from 0.6). It also makes the following plotting-related changes:

1. Default x and y margins are now back to 0.05. The old default of 0 looked pretty bad in a lot of situations (e.g. bar plots) and there wasn't really a great reason for this style change.
2. The area plot x-bounds are now "sticky", and I also made scalar y-bounds "sticky" so the autoscaling is similar to bar plots. Same goes for areax plot y-bounds and x-bounds.
3. Removes the 'rgbcycle' key (which was sort of a half-baked clone of seaborn's set_color_codes). A more proplot-ey idea would be to add an opencolor option that uses open-color red, blue, green, etc. for the color codes, but that's not high priority.
4. Removes the set_smart_bounds feature from FuncScale axis scales because it was deprecated in matplotlib.
5. Wrap pcolorfast just like pcolor and pcolormesh are wrapped. Wraps stem, vlines, and hlines so that x-coordinates and minimum y-coordinates are optional (resolves Property cycle support for "stem", "step", "vlines", and "hlines" #35).
6. Adds support for negpos bar plots, with negative blue bars and positive red bars (resolves Support for bar plots with "positive" and "negative" colors #34).
7. Renames add_errorbars to indicate_error and includes options for using shading to indicate the error bounds rather than error bars.