Add update_selection/1 to dynamically change the selection

[shamanime]

Closes https://github.com/maxmarcon/live_select/issues/68.

This allows updating the selection by passing a function with 1 arity to :update_selection.

It can be used to add/filter values from the existing selection without passing the whole selection as a value.

[maxmarcon]

Thanks @shamanime for your efforts! I will take a look at this soon