Support excluding of warnings via regex pattern

Example usage

  # plovr.cfg
  {
    "warning-exclude-paths": [
    "^/closure/goog/testing/stacktrace.js",
    "^/closure/goog/iter/iter.js",
    "^/closure/goog/labs/promise/promise.js",
    "^/closure/goog/labs/promise/thenable.js",
    # transitive dependencies seem to resolved more fully...
    ".*?/closure/goog/mochikit/async/deferred.js"
    ],
  }
Merge branch 'remotes/github/suppress-warnings'