Use strict TS

[lucasavila00]

Fixes #350

Notes: Using filenamify-url, unless in version 1.0.0, breaks the tests.
Version 1.0.0 does not export it's types, so we have to import them from @types/filenamify-url@1.0.0, and I don't know why it breaks the typescript compiler.

That's why I had to do this fix in index.ts

import * as _filenamifyUrl from 'filenamify-url';

 // TODO: update filenamifyUrl and fix the import
const filenamifyUrl = _filenamifyUrl.default;

[sindresorhus]

Thanks for working on this 🙌

Please take into account the feedback/reviews I've done in #356.

[sindresorhus]

Closing in favor of #356