Introduce Fetch#optional for performing optional fetches

[purrgrammer]

You can create Fetch values with Fetch#optional instead of Fetch#apply and you'll get back a Fetch[F, Option[A]] instead of a Fetch[F, A].

I still want to experiment working with optional fetches using OptionT but this change solves #45

• Update docs
• Add more testing

[codecov]

Codecov Report

Merging #159 into master will increase coverage by 0.61%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #159      +/-   ##
==========================================
+ Coverage    90.5%   91.12%   +0.61%     
==========================================
  Files           5        5              
  Lines         158      169      +11     
  Branches        5        5              
==========================================
+ Hits          143      154      +11     
  Misses         15       15

Impacted Files	Coverage Δ
shared/src/main/scala/fetch.scala	92% <100%> (+0.63%)	⬆️

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[peterneyens]

I don't think we need the cast here. Can we try Option.empty[A]?

[JorgeCastilloPrz]

Added to the ongoing redesign of Kollect.