Use DocStringExtensions in Gadfly

[tlnagy]

DocStringExtensions.jl is a really nice self-contained package that allows us to have much more flexible and dynamic docstrings. I'm very wary of adding more dependencies to Gadfly, but this package would add no subdependencies (it only depends on Compat and Base). A couple examples of where we can leverage it:

1. By using the automatically generated function signatures in the docstrings (so they don't get out sync) with the actual functions

2. By unrolling the fields in a type (see https://github.com/GiovineItalia/Gadfly.jl/pull/1157/files#diff-9ec506bf78232ae17d082c22c2e66449R507). Currently, all those docs are hidden when you do
   ?Gadfly.Stat.density, but could be unrolled with DocStringExtension.jl

Thoughts?

[bjarthur]

auto-gen'd sigs sounds great, but where besides #1157 are docs embedded in typedefs?

[bjarthur]

i can see the value in DocStringExtensions. #1173 proposes to use it.

[tlnagy]

Since it seems like we're generally onboard with using it, I'll go ahead and close this