update dependencies, remove lerna, fix tests

[chriswhong]

Do-over for #106

• Removes lerna altogether from the root of the repo, as older versions of it have security issues, and modern versions of it are not compatible with Mapbox npm publishing workflows.
• Adds a postinstall script to install all package dependencies. This allows a single npm ci command to install the top-level dependencies and the dependencies in each of the packages, which are necessary for linting and tests to pass.
• Updates a test and several snapshots

TODO:

1. First publish hast-util-to-jsx
2. Update jsxtreme-markdown to use the new version of hast-util-to-jsx
3. Publish jsxtreme-markdown
4. Update babel-plugin-transform-jsxtreme-markdown and jsxtreme-markdown-loader to use the new version of jsxtreme-markdown
5. Publish babel-plugin-transform-jsxtreme-markdown and jsxtreme-markdown-loader

[AndrewSepic]

Great. Thanks for implementing that postinstall.