Make ListItem image optional

It was supposed to be optional anyway, but setting it as undefined
to begin with makes it explicitly optional so Svelte doesn't start
giving warnings and stuff.

src/components/ListItem.svelte

@@ -2,7 +2,7 @@
   export let text = 'List Item'
   export let subtext = ''
   export let indicator = false
-  export let image
+  export let image = undefined
   export let tabindex = -1
   export let autofocus = false
   export let fullheight = false