support extending interfaces

[mattkrick]

Graphql can extend interfaces as well as types, eg extend interface Person. This was added just a short while ago: graphql/graphql-js#1222

i think all that's needed is to move the case up here, but I could be wrong: https://github.com/jimkyndemeyer/js-graphql-intellij-plugin/blob/accd108621006da46a0d1bea72b8efde3b1beda3/src/main/com/intellij/lang/jsgraphql/parser/JSGraphQLParser.java#L364

Thank you for the plugin! It's constantly saving me from myself.

[jimkyndemeyer]

The 2.0 alpha follows the latest GraphQL language draft spec as of June 2018.

See 2.0.0-alpha-2 if you'd like to try it and help test it.