Move JSHint options into an external .jshintrc file

While adding a `.jshintrc` admittedly created another mystery file that could conceivably confuse a user, it's the recommended way to configure JSHint and many editors can take advantage of these settings if they live within an external file.

Additional changes:

* Properties within `.jshintrc` have been sorted alphabetically.
* `jquery` and `browser` JSHint options have been set in .jshint, to reflect the most common JavaScript environment for WordPress developers
* `quotmark` JSHint option has been set to encourage consistency without enforcing an opinion on single- vs double-quotes
* The Gruntfile itself has been edited to pass `strict` and `trailing` JSHint tests by default, although I left those out of the default `.jshintrc` (along with many of the other more opinionated flags)