Add --old-implements-syntax option

[cjoudrey]

Closes #111

As of v0.13.0 of GraphQL.js the syntax for implementing multiple interfaces has slightly changed.

Previously you could separate each interface with a blank space or comma:

type Foo implements Bar, Baz { field: Type }

But the new syntax is to separate using &:

type Foo implements Bar & Baz { field: Type }

This pull request adds --old-implements-syntax in order to support schemas that still use the old way.

This option will be removed when GraphQL.js drops support for the old way.