Enable subplot axis label orientation

[Mattriks]

• I've updated the documentation to reflect these changes
• I've added and/or updated the unit tests
• I've run the regression tests
• I've built the docs and confirmed these changes don't cause new errors

This PR:

• enables changing the axis label orientation in subplots
• fixes Axis label orientation in subplots #424
• adds documentation about subplot inner and outer guides.

Example

using RDatasets
haireye = dataset("datasets","HairEyeColor")
palette = ["brown","blue","tan","green"]

plot(haireye, y=:Sex, x=:Freq, color=:Eye, ygroup=:Hair,
    Geom.subplot_grid(Geom.bar(position=:stack, orientation=:horizontal),
        Guide.ylabel(orientation=:vertical) ),
    Scale.color_discrete_manual(palette...),
    Guide.colorkey(title="Eye\ncolor"),
    Guide.ylabel("Hair color"), Guide.xlabel("Frequency")
)

[codecov-io]

Codecov Report

Merging #1424 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1424      +/-   ##
==========================================
+ Coverage   89.39%   89.42%   +0.02%     
==========================================
  Files          39       39              
  Lines        4318     4320       +2     
==========================================
+ Hits         3860     3863       +3     
+ Misses        458      457       -1     

Impacted Files	Coverage Δ
src/geom/subplot.jl	95.59% <100.00%> (+0.05%)	⬆️
src/guide.jl	89.22% <100.00%> (+0.22%)	⬆️

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[bjarthur]

LGTM. my only comment is that it would be nice to have the example in the docs be different than that in the tests. tests can double as docs, but only to the extent that they are different.

[Mattriks]

I thought the example was too nice 😃 I might add revamping subplot_grid to the next Roadmap (after #1385), to continue what I've started here, in which case I'll change/update the testscripts then.