Update retext, nlcst-to-textom
Refactor npm script targets in `package.json`
Fix incorrect executive rights on `test.js`
Rename project

`retext-ast` > `retext-cst`
Refactor `Readme.md`
Add flat badges to `Readme.md`
Refactor `index.js`
Remove exposed `toJSON` and `toCST` methods
Add support for an optional `options` object to `fromCST`