Rename FileInspector to Runner

Also rename #process_files to #run.
Move #display_error_summary from Runner to CLI

Runner should be unaware of UI. It's CLI's responsibility.
Handle TargetFinder in Runner rather than CLI

Also pass ConfigStore to Runner.new instead of Runner#run.
Return true when all passed in Runner#run

It's strange that returning true when something is bad.
Refactor handling of interrupt
Store encoding error in ProcessedSource and merge SourceParser into it

By storing the encoding error on parsing in ProcessedSource rather than
raising it, the offense emitting logic in Runner can be moved to
Cop::Team.

https://github.com/bbatsov/rubocop/pull/900#discussion-diff-10940125

Without merging the two classes, to store the error in ProcessedSource,
we would need to pass 6 arguments to ProcessedSource.new.
Also, since some of the attributes are dependent on others, it seems
that how the attributes are generated/dependent should be a knowledge of
ProcessedSource itself.
Extract syntax offense emitting logic in Runner to cop classes
Refactor elimination of duplicated offenses with Array#uniq!
Refactor failure level handling in Runner
Merge pull request #1166 from yujinakayama/refactor-core

Refactor core