Expand Scale sections in the Tutorial

[Mattriks]

• I've updated the documentation
• I've built the docs and confirmed these changes don't cause new errors

This PR:

• Expands the sections on Continuous and Discrete Scales in the Tutorial
• Inspired by Trouble using Scale.continuous_scale_partial with the color aesthetic #573 and Geom.bar - Colorkey does not adhere to the order specified in Scale.x_discrete #811
• closes Trouble using Scale.continuous_scale_partial with the color aesthetic #573

Note I leave #811 open, in case anyone wants to implement auto-updating Scale.color_discrete() if discrete aesthetic x==color.

[codecov-io]

Codecov Report

Merging #1245 into master will decrease coverage by 0.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1245      +/-   ##
==========================================
- Coverage   85.67%   85.49%   -0.19%     
==========================================
  Files          35       35              
  Lines        4085     4087       +2     
==========================================
- Hits         3500     3494       -6     
- Misses        585      593       +8

Impacted Files	Coverage Δ
src/ticks.jl	78.41% <0%> (-2.16%)	⬇️
src/aesthetics.jl	77.96% <0%> (-0.85%)	⬇️
src/guide.jl	83.17% <0%> (-0.49%)	⬇️
src/statistics.jl	90.41% <0%> (-0.21%)	⬇️
src/scale.jl	94.51% <0%> (+0.04%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update aa8dff9...9f439b6. Read the comment docs.

[tlnagy]

Note I leave #811 open, in case anyone wants to implement auto-updating Scale.color_discrete() if discrete aesthetic x==color.

I think it's reasonable to assume that if a discrete scale is chosen for x and color happens to have a 1 to 1 mapping to x then to assume the user also wants a color_discrete with the same parameters as x_discrete

[Mattriks]

I agree. Note that functionality is not implemented in ggplot2, as in my example, so it would be nice to have in Gadfly!

This present PR is just about Scale documentation - its ready to merge.

[tlnagy]

Note that functionality is not implemented in ggplot2

It's okay for us to pass ggplot2 thanks to all your hard work :)