Set search query as user types

When the user types in the text box, we set the query value on the
model, thus filtering the dropdown.

Note that the new Msg value `SearchInput` takes a paremeter, we don't
give it one in `onInput`. While `onClick` takes a Msg as its argument,
`onInput` takes a _function_ `String -> Msg` as its argument.

`SearchInput` is such a function when not given a value.

`onInput` does this because we don't know the value ahead of time.
Instead, it will put the whatever value the user has typed inside the
Msg.

Main.elm

@@ -1,7 +1,7 @@
 module Main exposing (main)
 
 import Html exposing (..)
-import Html.Events exposing (onClick)
+import Html.Events exposing (onClick, onInput)
 
 
 main =
@@ -58,6 +58,7 @@ viewOpen : Model -> Html Msg
 viewOpen model =
     div []
         [ dropdownHead model.selected CloseSelect
+        , input [ onInput SearchInput ] []
         , ul [] (List.map dropdownItem (filteredValues model))
         ]
 
@@ -98,6 +99,7 @@ type Msg
     = OpenSelect
     | CloseSelect
     | ItemSelected String
+    | SearchInput String
 
 
 update : Msg -> Model -> Model
@@ -111,3 +113,6 @@ update msg model =
 
         ItemSelected value ->
             { model | selected = Just value, state = Closed }
+
+        SearchInput query ->
+            { model | query = query }