Add `isSelectable` method

This commit adds an `isSelectable` callback which can be used to
indicate whether a given autocomplete suggestion should be
"selectable". This can be used to implements suggestion sections,
headings or similar.

Change-Id: Ibe88c57b021f0c3596415a8da320a4ef79aa9fe3
Reviewed-on: https://gerrit.causes.com/19125
Reviewed-by: Tony Wooster <tony.wooster@causes.com>
Tested-by: Tony Wooster <tony.wooster@causes.com>
Add `selectKeys` option

This commit adds a `selectKeys` option which is a customization that
allows a user to specify which keys should trigger selection. In some
situations, for example, pressing tab is expected, but pressing enter
is not or vice-versa. This aids in that use-case.

Change-Id: Ia744d87ea50bbf64873d9a08987c8d10b0fc1677
Reviewed-on: https://gerrit.causes.com/19126
Reviewed-by: Tony Wooster <tony.wooster@causes.com>
Tested-by: Tony Wooster <tony.wooster@causes.com>