Add map docs and examples

[davidanthoff]

Too rough to merge right now. Things that still need to happen:

• Wait for VegaDatasets.jl with path support to be tagged
• Make sure the one dot per zip map gets exported as png, not svg. Kills Chrome otherwise... This suggests we might need that as a per plot setting
• Add some more maps

[coveralls]

Coverage remained the same at 30.783% when pulling b570c09 on davidanthoff:map-doc-examples into 49ccbae on fredo-dedup:master.

[codecov-io]

Codecov Report

Merging #86 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #86   +/-   ##
=======================================
  Coverage   30.78%   30.78%           
=======================================
  Files          16       16           
  Lines         562      562           
=======================================
  Hits          173      173           
  Misses        389      389

Impacted Files	Coverage Δ
src/VegaLite.jl	82.75% <0%> (ø)	⬆️
src/dsl_helper_func/dsl_helper_func.jl	0% <0%> (ø)	⬆️

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