Precise issue location + Custom checks

This version is compatible with SonarQube 5.6+.
Note that analyses must be run with Java 8.

Release notes: https://github.com/racodond/sonar-json-plugin/milestone/5?closed=1

• Upgrade dependencies to SonarQube 5.6
• Now based on strongly typed AST
• Ability to write custom checks (see plugin example and detailed explanations at https://github.com/racodond/sonar-json-custom-rules-plugin)
• Precise issue location