Fix bugs, ability to set data-charlimiter-limit, option to bust character limit, customizable counter message, new callback (valid)

Changes include:
* Force maxChars to be set: Plugin is useless if no maxChars is found for a specific field. Exit plugin if this is the case.
* maxChars option can be set with the `maxlength` attribute or `data-charlimiter-limit`. This is important for the forceLimit option (detail below)
* Option for soft or hard limit: Added the `forceLimit` option. This allows a user to continue writing when the limit is passed. Sites like Twitter does something like this. The callback can be used to block the submit button. This helps for copy-pasting things.
* Customizable output message: The counter message can contain lots of different messages instead of just a remaining number. See readme for more details.
* Pass element in callback: The element is now passed in the callback.
* Added a callback: The callback is called when user is within limits.
* Update README.md