Support disable directives

[DonIsaac]

Partially implemented by #169 (directive parsing) and #172 (global directives). Line-specific directives still need to be implemented. All the pieces exist

AC

• Ignore checks should be done only when a violation is reported.
  • hypothesis: directives aren't common, and lint violations are even less common
• Directives should only be parsed when a comment is found above a diagnostic. No parsing should happen before its needed
• Parse results should be cached. Do not parse the same comment twice.