Add "zlintConfig" to cert-checker

[aarongable]

The CA (really the issuance package) has two config keys which affect its linting:

• IgnoredLints
• LintConfig

The cert-checker only has the first of these. This means that it cannot configure our custom pkilint check. We'd like to start running pkilint (or pkiemetal) in prod, but only as part of cert-checker for now. Therefore we need cert-checker to be able to load a zlint config file to enable and configure our custom lint.