feat(enter): make enter work inside form

[codiemullins]

What: Allow enter key to submit form, if downshift is inside a form tag.

Why: So form can be submitted with enter key if focus is on downshift input.

How: Moved event.preventDefault() so it only executes if menu is open

Checklist:

• Documentation - N/A
• Tests (kind of, added a story book... may need unit test? Wasn't sure)
• Ready to be merged
• Added myself to contributors table

[kentcdodds]

Most excellent! Thank you very much for this!

[codecov-io]

Codecov Report

Merging #161 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #161   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           4      4           
  Lines         267    267           
  Branches       64     64           
=====================================
  Hits          267    267

Impacted Files	Coverage Δ
src/downshift.js	100% <100%> (ø)	⬆️

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