Add "properties" to all matplotlib artists

[lukelbd]

This PR is related to #89 and adds @property setters and getters to every single Artist subclass. The core logic for this turns out to be surprisingly simple:

https://github.com/lukelbd/proplot/blob/5d29f3148975e898452c4101dab6338e2908c50f/proplot/artist.py#L14-L50

After this PR is merged proplot users will be able to use the much cleaner, more object-oriented "dot" syntax for changing matplotlib artist attributes:

# Original API
fig, ax = pyplot.subplots()
ax.set_title('Title')
ax.set_xlabel('xlabel')
ax.set_xticks([1, 2, 3])
h, = ax.plot(...)
h.set_color('red')
h.set_linewidth(1)


# ProPlot API if this is merged
fig, ax = proplot.subplots()
ax.format(title='Title', xlabel='xlabel', xticks=[1, 2, 3])  # with bulk setter
ax.title = 'Title'  # with individual setters
ax.xlabel = 'xlabel'
ax.xticks = [1, 2, 3]
h, = ax.plot(...)
h.color = 'red'
h.linewidth = 1

I am not sure about the side-effects of this -- the problem is this breaks classes that use instance-level attributes with the same names as the setters and getters (e.g. if an artist subclass both the getter get_linewidth() and the literal attribute linewidth), and the only way to address this is to go through the source code class-by-class and make explicit exceptions (which is quite fragile, since the attributes may change from version-to-version). Monkey patching is not possible because you cannot add properties as instance-level attributes.

This is a really neat PR, but it will require a lot of testing and it's not as high priority as some other PRs. So it may be quite a while before this is merged.