support : and ? in flow context as per updated YAML spec

[Herringway]

Reviving #324. not sure why I thought it was wrong, but I was wrong about being wrong

[codecov]

Codecov Report

Merging #328 (3834e04) into master (d009f15) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #328      +/-   ##
==========================================
- Coverage   93.74%   93.74%   -0.01%     
==========================================
  Files          29       29              
  Lines        4573     4570       -3     
==========================================
- Hits         4287     4284       -3     
  Misses        286      286              

Files	Coverage Δ
source/dyaml/scanner.d	99.65% <100.00%> (-0.01%)	⬇️

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