Add description to schema parser and printer

[phil303]

Descriptions are now part of the graphql spec. This commit brings graphql-core inline with the spec.

There's an additional commit in here that fixes the indent function in the printer. It's not a requirement for handling descriptions.

One note, the business logic in here is heavily lifted from the JS reference implementation.

[phil303]

@jkimbo @syrusakbary Hey guys, what can I do to move this PR forward?

[gizmo385]

@jkimbo @syrusakbary +1

This feature would be really useful as other GraphQL libraries in other languages support descriptions.

[waxlamp]

Another vote for getting this merged (cc @jkimbo and @syrusakbary). Thanks for this, @phil303!