fix(label): support aria-label

[kentcdodds]

What: This adds support for aria-label in getByLabelText API

Why: Because sometimes you have an input which has styling to suggest its purpose (a visible label is unnecessary) and can use the aria-label for users who are using a screen reader.

How:

• Change the queryByPlaceholder to a general queryByAttribute (not exposed) and use that for queryByPlaceholder, queryByTestId, and inside queryByLabelText.
• Add a test to verify the aria-label support
• Update the docs

Checklist:

• Documentation
• Tests
• Ready to be merged
• Added myself to contributors table N/A

[codecov]

Codecov Report

Merging #42 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #42   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           5      5           
  Lines          85     85           
  Branches       19     19           
=====================================
  Hits           85     85

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

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