Custom focus implementation

[TheRusskiy]

Addresses https://github.com/final-form/final-form-focus/issues/13

Example usage:

const focus = (input) => {
  input.focus({ preventScroll: true })
  if (input.scrollIntoView) {
    input.scrollIntoView({
      behavior: 'smooth',
      block: 'center',
    })  
  }
}
createDecorator(undefined, undefined, focus)

[codecov[bot]]

Codecov Report

Merging #25 (48f7dfb) into master (c26b46e) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #25   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         6    +1     
  Lines           44        48    +4     
  Branches        11        12    +1     
=========================================
+ Hits            44        48    +4     

Impacted Files	Coverage Δ
src/decorator.js	100.00% <100.00%> (ø)
src/focusInput.js	100.00% <100.00%> (ø)

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[ricmello]

Hey guys! Any chances of getting this PR merged?