List: Expose setting of FilterValue and triggering filtering programmatically

Hey guys,
when using list to show and filter a list of items it would be handy to be able to not only set items programmatically, but also set the `FilterValue` and triggering filtering programmatically.

Currently we have:

```
// FilterValue returns the current value of the filter.
func (m Model) FilterValue() string {
return m.FilterInput.Value()
}
```

so the FilterValue is directly tied to the `FilterInput` . I could theoretically hand everything down to the `FilterInput`, but that would be very tedious, when I have an external separate TextInput which might set the filter.

Especially, when the filterInput sets this:

`filterInput.Prompt = "Filter: "` - so there is no way to customize this.

So my wishlist would be:

- [ ] Add a `SetFilterValue`-func
- [ ] Add a `SetLiveFilteringEnabled`-func which determines if the list is automatically filtered via `FilterValue`
- [ ] Add a `Filter`-func that actually triggers filtering via `FilterValue`

Thanks for the awesome work!