Adds an initial grammar for the remarkable Gravity language to Atom.
Initially ported from https://github.com/Dohxis/vscode-gravity with modifications.
Note: As Gravity is still a rapidly evolving language and the grammar definition is in flux, this package may be inaccurate or become out of date quickly. If you notice any issues, I'd be grateful if you'd open an issue in the repository or send a pull request. Thanks!
