Update gql-merge multiple inheritance syntax

[heylookalive]

Hi, I'm using gql-merge and when trying to run it against a schema which has multiple inheritance via the updated syntax I'm getting an error:

> ./node_modules/gql-merge/dist/index.js -v -o ./src/appsync/schema.graphql ./src/appsync/graphql/**/*.graphql

(node:83712) UnhandledPromiseRejectionWarning: GraphQLError: Syntax Error GraphQL (2:35) Cannot parse the unexpected character "&".

1: 
2: type CreativeWork implements Node & Output {
                                     ^
3:   id: ID!

    at syntaxError (/Users/xxxxxx/Sites/aaah/xxxxxx/node_modules/graphql/error/syntaxError.js:28:15)
    at readToken (/Users/xxxxxx/Sites/aaah/xxxxxx/node_modules/graphql/language/lexer.js:247:32)
    at Object.advanceLexer [as advance] (/Users/xxxxxx/Sites/aaah/xxxxxx/node_modules/graphql/language/lexer.js:46:28)
    at expect (/Users/xxxxxx/Sites/aaah/xxxxxx/node_modules/graphql/language/parser.js:963:11)
    at parseName (/Users/xxxxxx/Sites/aaah/xxxxxx/node_modules/graphql/language/parser.js:88:15)
    at parseNamedType (/Users/xxxxxx/Sites/aaah/xxxxxx/node_modules/graphql/language/parser.js:577:11)
    at parseImplementsInterfaces (/Users/xxxxxx/Sites/aaah/xxxxxx/node_modules/graphql/language/parser.js:702:18)
    at parseObjectTypeDefinition (/Users/xxxxxx/Sites/aaah/xxxxxx/node_modules/graphql/language/parser.js:681:20)
    at parseTypeSystemDefinition (/Users/xxxxxx/Sites/aaah/xxxxxx/node_modules/graphql/language/parser.js:607:16)
    at parseDefinition (/Users/xxxxxx/Sites/aaah/xxxxxx/node_modules/graphql/language/parser.js:148:16)
    at parseDocument (/Users/xxxxxx/Sites/aaah/xxxxxx/node_modules/graphql/language/parser.js:106:22)
    at parse (/Users/xxxxxx/Sites/aaah/xxxxxx/node_modules/graphql/language/parser.js:43:10)
    at mergeString (/Users/xxxxxx/Sites/aaah/xxxxxx/node_modules/gql-merge/dist/index.js:263:39)
    at mergeStrings (/Users/xxxxxx/Sites/aaah/xxxxxx/node_modules/gql-merge/dist/index.js:254:10)
    at _callee$ (/Users/xxxxxx/Sites/aaah/xxxxxx/node_modules/gql-merge/dist/index.js:45:46)
    at tryCatch (/Users/xxxxxx/Sites/aaah/xxxxxx/node_modules/babel-runtime/node_modules/regenerator-runtime/runtime.js:62:40)
(node:83712) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:83712) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

graphql/graphql-js#1169