Make resolvers optional

[lorensr]

Fixes #273, matching our docs quoted here: #273 (comment)

[codecov-io]

Codecov Report

Merging #275 into master will decrease coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #275      +/-   ##
==========================================
- Coverage   95.71%   95.65%   -0.07%     
==========================================
  Files           2        2              
  Lines          70       69       -1     
  Branches       22       24       +2     
==========================================
- Hits           67       66       -1     
  Partials        3        3

Impacted Files	Coverage Δ
packages/apollo-link-state/src/index.ts	94.82% <100%> (-0.09%)	⬇️

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