update docs to reflect that `container` option can be JQuery

I am currently passing a `JQuery` element as the `container` argument when instantiating: http://amsul.ca/pickadate.js/date/#container

This is important since we have a reusable widget, so any selector within its scope will return multiple elements. However, `localVariable.find(selector)` will return a valid `JQuery` instance.

Assuming this is supported (it works currently since the pickadate source code accesses `container` via `$`), can you please add it to the docs so that we can be sure we're using the library in a prescribed manner?

I'll also update the typescript definitions, provided this is supported.

Thanks!