Check response Content-Type

Instead of errors such as:

    invalid character '<' looking for beginning of value

Try to give slightly more useful errors with the HTTP error,
if any.
cmd/gqlclientgen: default pkg name to dirname

Arguably more useful than "main".
cmd/gqlclientgen: error out on nested aliases

We don't support them right now. Error out if we hit any.
cmd/gqlclientgen: collect fragments

If a query uses named fragments, extract them and include them.
Improve doc comment formatting
cmd/gqlclientgen: handle descriptions
Turn object/interface fields into pointers

This is required to deal with recursive types such as:

    type T {
        t T!
cmd/gqlclientgen: add support for unions
cmd/gqlclient: new CLI utility
readme: add build status badge
ci: add .build.yml
readme: add contributing section
readme: add link to GoDoc examples
Add json struct field tags

This makes it so input objects are correctly marshalled.
Generate structs for input objects
Add GraphQL query code gen
readme: add usage section
cmd/gqlclientgen: add flags

We'll add more input knobs soon, so positional args are getting

While at it, add support for multiple schemas.
Extract ast.Definition generation to function
Add disclaimer to generated file