left and right triangles

[Mattriks]

• I've added and/or updated the unit tests
• I've run the regression tests
• I've updated the documentation to reflect these changes
• I've squash'ed or fixup'ed junk commits with git-rebase
• I've built the docs and confirmed these changes don't cause new errors

This PR:

• adds left and right triangles

Example

shapes = [Shape.ltriangle; Shape.rtriangle; Theme().point_shapes]
D = DataFrame(x=repeat([1:5;], outer=3), y=repeat([3,2,1], inner=5), g=string.(1:15))

coord = Coord.cartesian(xmin=0, ymin=0, xmax=6, ymax=4)
p = plot(D, coord, 
    layer(x=:x, y=:y, Geom.point, Theme(default_color="black", point_size=2px)),
    layer(x=:x, y=:y, Geom.point, shape=:g, color=:g),
    Theme(point_shapes=shapes, key_position=:none, point_size=12pt, 
        plot_padding=[0mm], background_color="white")
)

[Mattriks]

On Travis, julia 1.0 fails on a test (discrete_color_hue.jl) that seems unrelated to this PR.

[codecov-io]

Codecov Report

Merging #1365 into master will increase coverage by 0.39%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #1365      +/-   ##
=========================================
+ Coverage    90.6%     91%   +0.39%     
=========================================
  Files          38      38              
  Lines        3961    4535     +574     
=========================================
+ Hits         3589    4127     +538     
- Misses        372     408      +36

Impacted Files	Coverage Δ
src/theme.jl	69.49% <ø> (ø)	⬆️
src/shapes.jl	100% <100%> (ø)	⬆️
src/ticks.jl	90.32% <0%> (-1.49%)	⬇️
src/statistics.jl	96.28% <0%> (-0.6%)	⬇️
src/aesthetics.jl	83.15% <0%> (+1.26%)	⬆️
src/coord.jl	93.66% <0%> (+2.26%)	⬆️

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