Allow binders to be chained together to create multi-source binder

[aldas]

There are (rare) cases when you would want to bind value from different sources. So this PR allows binders to be chained together to create multi-source binder.

This has few use-cases because we have ValueBinders only for Path/Query/Form and most of the time if multi-source binding is needed we talk about body (ala JSON) + some other source (ala query).

Example:

// bound query params should have priority over path params
b := QueryParamsBinder(c).UseBefore(PathParamsBinder(c))

or

// bound params priority:
// 1. Path params
// 2. Query params
// 3. Form fields
b := PathParamsBinder(c).UseBefore(QueryParamsBinder(c)).UseBefore(FormFieldBinder(c))

---

I am not sure if this name UseBefore is best. I considered Combine, CombineWith, CombineBefore, Use, Chain

*Before is added as suffix to make it clear there is order of binding. *Before indicates that left side binder value is used before right side binder value. But I am at the moment no even sure that people would read/understand it like I do.

I am open to suggestions for naming it.

p.s. this is low priority, low value change

[codecov[bot]]

Codecov Report

Base: 92.35% // Head: 92.38% // Increases project coverage by +0.02% :tada:

Coverage data is based on head (510f12d) compared to base (8f2bf82). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2311      +/-   ##
==========================================
+ Coverage   92.35%   92.38%   +0.02%     
==========================================
  Files          37       37              
  Lines        4436     4450      +14     
==========================================
+ Hits         4097     4111      +14     
  Misses        247      247              
  Partials       92       92              

Impacted Files	Coverage Δ
binder.go	100.00% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

:umbrella: View full report at Codecov.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.

[aldas]

@lammel when you have time. This is low priority PR, so no pressure here :)