MEI language support in Atom

Adds syntax highlighting to MEI files in Atom and provides the mei-all.rng schema for validation and autocomplete with the linter-autocomplete-jing package.

Originally copied from the XML language package.

Additional MEI-specific features will (probably) be added in the future.

Contributions are greatly appreciated. Please fork this repository and open a pull request to add snippets, make grammar tweaks, etc.