Readme: Add Plugins section

Closes gh-761
Fixes #760
Docs: Added clarification of tokens in disallowSpacesInConditionalExpression

Closes gh-763
Docs: Typo fix

Closes gh-766
New Rule: disallowOperatorBeforeLineBreak

Fixes #511
Closes gh-754
requireCapitalizedComments: Better support for multi-line comments

Closes gh-769
Fixes #767
Errors: Support a filter to control which errors are reported

Closes gh-728
Fixes #335
Preset: Grunt

Fixes #742