Introduce force framework and convert VariableInspector into a force

The force framework allows cops to share common logic and reduce wasted
CPU power by module mixins. A force implements custom processing and
invokes some hooks in cops that belong to the force.
[#934] New cop UnderscorePrefixedVariableName
Fix an offense reported by UnderscorePrefixedVariableName
Merge pull request #979 from yujinakayama/underscore-prefixed-variable-name

New cop UnderscorePrefixedVariableName