StringChecker: unsupported rules shown as style errors and not thrown exceptions

Closes gh-710
Fixes #334
disallowMultipleVarDecl: add exception for undefined variable declarations

Closes gh-749
Fixes #451
Update unit-coverage to 3.2.0
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