Add basic support for typescript files

[zimme]

This should allow .ts/.tsx files to be run through prettier and eslint with proper parser settings.

[codecov-io]

Codecov Report

Merging #121 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #121   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           2      2           
  Lines         187    191    +4     
  Branches       31     32    +1     
=====================================
+ Hits          187    191    +4

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

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[zimme]

I don't know if better tests are needed. I've had a hard time validating that the typescript file is actually ran through eslint and fixed in any way.

[kentcdodds]

Looks great! Thanks!

[kentcdodds]

Thanks so much for all your help! I've added you as a collaborator on the project. Please make sure that you review the CONTRIBUTING.md file (specifically the bit about the commit messages and the git hooks) and familiarize yourself with the code of conduct (we're using the contributor covenant). You might also want to watch the repo to be notified when someone files an issue/PR. Please continue to make PRs as you feel the need (you can make your branches directly on the repo rather than your fork if you want). Thanks! And welcome to the team :)