Add ability to run summaries

[raulpopadineti]

This PR adds the ability to run a summary on all javascript files from the project and display the number of warnings either grouped by filename or by warning (type).

I've also included a summary per file where you can see the number of warnings you have on a certain JS/JSX.

It's a very similar output to what Rubocop does with the offenses formatter (http://rubocop.readthedocs.io/en/latest/formatters/#offense-count-formatter).

I think it's a good way of having a glimpse on what to focus on first, when getting your hands on a bigger project.

Here are a few sample outputs of the commands:

Full summary examples (by filename and warning)

$ rake eslint:summary[filename]
517       app/assets/javascripts/lib/rollbar.js
177       app/assets/javascripts/lib/nprogress.js
6         app/assets/javascripts/homepage.js
6         app/assets/javascripts/card_form.js
6         app/assets/javascripts/application.js
6         app/assets/javascripts/board.js
6         app/assets/javascripts/test_bundle.js
--
724 warning(s) found.

$ rake eslint:summary[warning]
121       VariableDeclarator/space-infix-ops
108       Punctuator/comma-spacing
69        Literal/quotes
54        Identifier/no-shadow
45        ReturnStatement/semi-spacing
44        Identifier/no-undef
35        VariableDeclaration/semi
31        VariableDeclarator/no-underscore-dangle
25        IfStatement/curly
23        Identifier/no-use-before-define
22        Program/no-trailing-spaces
19        SequenceExpression/no-sequences
19        ExpressionStatement/no-unused-expressions
13        BinaryExpression/eqeqeq
9         ObjectExpression/key-spacing
8         BinaryExpression/yoda
8         BinaryExpression/space-infix-ops
7         Program/no-empty-class
7         Program/no-extra-strict
7         Punctuator/no-multi-spaces
7         Program/no-empty-label
7         Program/space-return-throw-case
7         Program/no-wrap-func
7         Program/global-strict
5         CatchClause/no-catch-shadow
3         ExpressionStatement/strict
2         ExpressionStatement/semi
2         Identifier/no-unused-vars
2         NewExpression/new-parens
2         NewExpression/new-cap
2         BlockStatement/no-empty
1         EmptyStatement/no-extra-semi
1         MemberExpression/no-console
1         FunctionExpression/consistent-return
1         ForStatement/no-cond-assign
--
724 warning(s) found.

File summary (by default for application.js)

$ rake eslint:file_summary
1         Program/no-empty-class
1         Program/no-empty-label
1         Program/no-extra-strict
1         Program/no-wrap-func
1         Program/global-strict
1         Program/space-return-throw-case
--
6 warning(s) found.

[raulpopadineti]

@jonkessler can you have a look at this and let me know if it needs anything else?