Update keywords
Update dependencies
Test with `deepEqual()`

PostCSS's `process()` and `toResult()` returns `SourceMapGenerator`
Object, not String.
Version 0.2.1
Specify a license with SPDX ID
Merge commit '1f7a92fbd120143846e98c768615686ac33b805e'

* commit '1f7a92fbd120143846e98c768615686ac33b805e':
  Specify a license with SPDX ID
  Version 0.2.1
  Test with `deepEqual()`
  Update dependencies
  Update keywords
Add tests for the new instance interface.
Rename .processor to `.postcss`

This matches the name that autoprefixer and all other postcss based libraries use.

For backwards compatibility `csswring.processor` is retained as an alias.