Thunk-ify layout options

[jonjohnsonjr]

If we want to add anything other than descriptor mutators as options, we
need a place to do that, so this introduces an options struct that holds
options and thunk-ifies the existing descriptor mutators.

[codecov-io]

Codecov Report

Merging #847 (24c04e1) into master (cf9d7b3) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #847      +/-   ##
==========================================
+ Coverage   74.72%   74.84%   +0.11%     
==========================================
  Files         106      106              
  Lines        4427     4432       +5     
==========================================
+ Hits         3308     3317       +9     
+ Misses        629      627       -2     
+ Partials      490      488       -2     

Impacted Files	Coverage Δ
pkg/v1/layout/options.go	100.00% <100.00%> (ø)
pkg/v1/layout/write.go	51.58% <100.00%> (+3.17%)	⬆️

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[deitch]

I like this (but you knew that). Looking forward to expanding this once it is in.