Add `shouldFocusInput` option [Fix #1541]

This adds a `shouldFocusInput` option that determines whether or not the focusser should be automatically focused.  By default, the focusser will always be focused, unless `minimumResultsForSearch` is less than zero.

`close` on single selects took an option `params` argument which at one point was used to tell the focusser to not automatically focus.  This was [added at one point](https://github.com/ivaynberg/select2/commit/e162a4802cddfb95fdef8793fc83fc07897650ec) when tweaking the mask and [later reverted](https://github.com/ivaynberg/select2/commit/9bc68f089e4d81e0dd0e8de943071ae36a3394cd) to fix a bug that came as a result [2].  The leftovers of this code has been removed in this commit.