validator: make rules exported and allow custom set of rules for Validate

[kgrigorev]

validator.Validate function uses predefined set of rules, which are populated during package initialisation. While it is possible to add custom rules via AddRule, it is not possible to have completely customised set of rules (e.g. if someone wants to delete or replace some predefined rule). The javascript implementation allows to pass custom set of rules and falls back to the default set only if the parameter is null (https://github.com/graphql/graphql-js/blob/bd2fe71d2a7ca8619ec71bae919692ce61e10958/src/validation/validate.ts#L40). This PR converts every rule to the package-level variable and allows to pass custom set of rules to the Validate function as varargs. It also falls back to predefined set of rules for the backward compatibility, so that none of the existing code is broken. With that change validation can be customized:

errList := validator.Validate(s, q, []validator.Rule{rules.FieldsOnCorrectTypeRule, rules.VariablesInAllowedPositionRule}...)

Describe your PR and link to any relevant issues.

I have:

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

[coveralls]

coverage: 88.168% (+0.1%) from 88.038%
when pulling a4cc52c on kgrigorev:export_validator_rules
into ed10d5c on vektah:master.

[StevenACoffman]

Thanks! One thing to note is that changing the package variable will not be safe for tests that run in parallel, as well as in production code.