Add formatter package

[vvakame]

Motivation

I'm trying to genarate gRPC <-> gqlgen glue code. (in vvakame/til#31)
I wanna generate 1. part of resolver implementation, 2. part of schema.graphql from .proto file.

genarate schema.graphql is not easy.
but construct ast/SchemaDocument is easy (maybe...).

Goal

• convert AST to schema or query text

Non-goal

• configurable
• re-emit same as original source order
• add extra new package
  • e.g. internal/walker that shared with validator

#101 is same request? 👀

[coveralls]

Coverage increased (+0.9%) to 92.298% when pulling 9f6da28 on vvakame:feat-formatter into 05741cd on vektah:master.

[vvakame]

🤔 My test code covered all statement (without return err) more than once...