Fixes selection and disable toolbar behaviors

- Toolbar is created once.
- Bind `mousedown` to elements to detected where the selection started.
- Fix checkSelection and prevent toolbar to open in a invalid editor.
- Display block on toolbar when activate() is called

disableToolbar and data-disable-toolbar behavior:
- If `disableToolbar` is true on options, all elements in that instance of mediumEditor will have toolbar disabled. (no toolbar will be created)
- if `disableToolbar` is false on options:
 - if all elements have `data-disable-toolbar` toolbar will be disabled in each one (no toolbar will be created)
 - if any element dont have `data-disable-toolbar`, a toolbar will be created and will work only on that element.