Allow fragment name to be optional

[devknoll]

This would allow Relay fragments (which omit the name) to be parsed correctly.

https://github.com/graphql/codemirror-graphql/blob/master/src/mode.js#L382

[leebyron]

The codemirror mode now parses correctly, though the linter is still likely to complain about a syntax error since it's technically not valid GraphQL.