Missing and NaN support

[Mattriks]

This PR

• For lines with missing or NaN, makes SVG images consistent with Cairo
• fixes Missing values #384

Example

y = [0.26, 0.5, missing, 0.4, NaN, 0.48, 0.58, 0.83]
p1 = collect(Tuple, zip(1:8, y))
p2 = collect(Tuple, zip(1:9, vcat(NaN, y)))

p = compose(context(units=UnitBox(0,0, 10, 1)), 
    (context(), rectangle(), fill(nothing), stroke("gray")),
    (context(), line([p1]), stroke("black")),
    (context(), line([p2]), stroke("red"))  
)
# draw(SVG(), p) # SVG and PNG look the same
draw(PNG(), p)

[codecov-commenter]

Codecov Report

Merging #397 into master will increase coverage by 0.17%.
The diff coverage is 96.42%.

@@            Coverage Diff             @@
##           master     #397      +/-   ##
==========================================
+ Coverage   44.07%   44.24%   +0.17%     
==========================================
  Files          18       18              
  Lines        3385     3390       +5     
==========================================
+ Hits         1492     1500       +8     
+ Misses       1893     1890       -3     

Impacted Files	Coverage Δ
src/Compose.jl	31.39% <ø> (ø)
src/svg.jl	72.47% <96.15%> (+0.42%)	⬆️
src/measure.jl	51.44% <100.00%> (+1.44%)	⬆️

---

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 6777e8f...1f96ae3. Read the comment docs.