choo components (#639)

* add components

* example: componentize header

* fix choo SSR

* example: componentize footer

* example: fix footer

* update component cache asserts

* componentize todos

* reorder example dir

* fix example tests

* fix dep check test

* Add garbage collection of unused components

* Apply args when calling identity

* update to new component preview

* update nanocomponent

* fix cache err name

* remove static methods from example

* restore app.emit() function

* public API tests

* offset component cache iteration by 2

* whitelist contents of component folder (#643)

* allow lru number arg

* fix lint typo

example/components/footer/clear-button.js

@@ -0,0 +1,15 @@
+var html = require('bel')
+
+module.exports = deleteCompleted
+
+function deleteCompleted (emit) {
+  return html`
+    <button class="clear-completed" onclick=${deleteAllCompleted}>
+      Clear completed
+    </button>
+  `
+
+  function deleteAllCompleted () {
+    emit('todos:deleteCompleted')
+  }
+}