Set `navOpen` state in the `_init` method

Set the `navOpen` state at the same time as the "closed" class is added,
i.e. in the `_init` method.

Fixes the issue where:
- initializing a new instance: `var nav = responsiveNav('#nav')`
- toggling it (to `navOpen = true`): `nav.toggle()`
- destroying it: `nav.destroy()`
- re-initializing it: `nav = responsiveNav('#nav')`
Would cause the second initialization to have conflicting state
`navOpen === true` and the "closed" class on the DOM element.