Exposes layout object to validators

[cperryk]

This change passes the layout object to validation functions so we can develop layout-specific rules. (Relevant Trello)

[coveralls]

Coverage increased (+1.1%) to 73.087% when pulling 9554a5f on layout-in-validation into d8c7431 on master.

[jonwinton]

This would also require an update of documentation: https://github.com/nymag/clay-kiln/blob/master/validators/README.md

[coveralls]

Coverage increased (+1.03%) to 72.999% when pulling e1bf878 on layout-in-validation into d8c7431 on master.

[nelsonpecora]

👍🏼 from sunny CA

[nelsonpecora]

note: in kiln v1.0, we're passing the full state object, so you can grab the layout uri (state.page.data.layout) or data (state.components[layoutURI]) in the validator yourself.