Adding guides to a blank plot

[Mattriks]

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

This PR:

• Make adding guides to a blank plot work more generally e.g. plot(Guide.title("My Awesome Plot"))
• Includes a doc example of adding guides to a blank plot (cf. spacings in hstack/vstack/gridstack #1169 (comment))

[codecov-commenter]

Codecov Report

Merging #1449 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1449   +/-   ##
=======================================
  Coverage   89.40%   89.40%           
=======================================
  Files          39       39           
  Lines        4379     4379           
=======================================
  Hits         3915     3915           
  Misses        464      464           

Impacted Files	Coverage Δ
src/guide/keys.jl	82.69% <ø> (ø)
src/Gadfly.jl	76.00% <100.00%> (ø)

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