Sphinx Text language support in Atom

Adds syntax highlighting and snippets to Sphinx files in Atom.

Originally converted from the ReStructuredText Atom extension https://github.com/Lukasa/language-restructuredtext.

Contributions are greatly appreciated.

Hints for improving the grammar

• The Atom package atom-live-grammar-reload really facilitates the tedious fine tuning.

• Some hints on naming conventions: https://manual.macromates.com/en/language_grammars.html