Add targetEls Option that Configures Specific input Elements for Update

Sorry about the duplicate issue. This is work for [issue 198](https://github.com/tarruda/bootstrap-datetimepicker/issues/198).

Adds `targetEls` configuration option that can either be an element selector `string` or jQuery object. Added `getTargetEls` function to the prototype that returns based on the presence of a truthy `jquery` value on `options.targetEls`. Defaults to `input` so behavior is unchanged.

I haven't written unit tests due to [issue 200](https://github.com/tarruda/bootstrap-datetimepicker/issues/200) (an issue with the Bootstrap submodule).