Introduce Bootstrap

[vkostyukov]

This suppresses #583 and fixes #575.

The idea is pretty much the same. Here is the syntax example based on our TE benchmark:

val service = Bootstrap
  .serve[Application.Json](json)
  .serve[Text.Plain](plaintext)
  .toService

The reason I decided to call it Bootstrap is that I believe we would be able to add more machinery to it so users can configure a bunch of other things in their derived services. Examples:

• Report stats per endpoint (into a given stats receiver), see Metrics for endpoints #781
• Enable responding 405, see Support 405 #784

This new Bootstrap type can also act as a config for our existing features. For example, we might think of providing a way to disable the Date header, supplied by default. Something along the lines (just using the case class copy method, somewhat close to what Circe does with Printers):

val service = Bootstrap.copy(includeDateHeader = false, includeServerHeader = true)
  .serve[Application.Json](foo)
  .toService

This is an experimental API and I'm not deprecating anything. The goal is to solicit users' feedback in 0.16 and see if it gets any adoption.

[codecov-io]

Codecov Report

Merging #794 into master will increase coverage by 0.27%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #794      +/-   ##
==========================================
+ Coverage   79.17%   79.45%   +0.27%     
==========================================
  Files          44       45       +1     
  Lines         682      691       +9     
  Branches       30       22       -8     
==========================================
+ Hits          540      549       +9     
  Misses        142      142

Impacted Files	Coverage Δ
...e/src/main/scala/io/finch/internal/ToService.scala	100% <100%> (ø)	⬆️
core/src/main/scala/io/finch/Endpoint.scala	73.07% <100%> (ø)	⬆️
core/src/main/scala/io/finch/Bootstrap.scala	100% <100%> (ø)

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