Supporting directive on the schema

[hori0926]

This enables gqlparser to support directives on a schema, which is a part of the GraphQL specification. This fixes #260

I have:

• Added tests covering the bug / feature
• N/A Updated any relevant documentation

[coveralls]

coverage: 88.017% (-0.02%) from 88.038%
when pulling 6074e71 on hori0926:directive-on-schema
into ed10d5c on vektah:master.

[hori0926]

I have taken a brief look at the CI failure, and it appears that this failure is not caused by the changes in this pull request. The CI has been failing even before this pull request was submitted. Given this situation, how should we proceed to address this CI failure? What would be the most appropriate course of action for us to take?

[StevenACoffman]

Hi, yeah, that's not your problem to fix. When we upgraded the linter, the rules started to flag those use of string variables as format strings but it's not really a problem, since they display fine.

I'm a little swamped at work, and every time I start reading through the GraphQL spec to verify that this is entirely correct, I get interrupted. Sorry for the delay.

[hori0926]

Thank you very much for your response. I truly appreciate your assistance.
I apologize for taking up your valuable time, but I would be grateful if you could continue to work with me on this.
No worries about the delay!

[StevenACoffman]

Thanks for your patience! I appreciate you working on this, and look forward to your next contribution.

[hori0926]

Thank you for your response and for merging my contribution!
I look forward to continuing to work together.