Split the graph schema file in one file per object

[davecaos]

Add the possibility to split the graph schema in diferentes files, that files must be joined and evaluated as a singe schema file.
As @jlouis suggested , we could use a directive at top of file like package my-graphql-erlang, for example where every file that references that package will be into my-graphql-erlang.graphql

[jlouis]

This has relevance to #183 where the type extension language is being made explicit.