Allow focus on list filter input.

Show filter input only where necessary, and permit it to be focussed.
Make backspace focus input in single-select mode.

When a single tag is selected in single-select mode, it may still be
focussed and backspace will remove it (as per multi-select mode). In
addition to doing this, the backspace key should cause the input to
receive focus.
Merge pull request #24 from benbasson/fix-filter-focus

Allow focus on list filter input.