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/header.js

@@ -0,0 +1,32 @@
+var Component = require('../../component')
+var html = require('bel')
+
+module.exports = class Header extends Component {
+  constructor (name, state, emit) {
+    super(name)
+    this.state = state
+    this.emit = emit
+  }
+
+  update () {
+    return false
+  }
+
+  createElement () {
+    return html`<header class="header">
+      <h1>todos</h1>
+      <input class="new-todo"
+        autofocus
+        placeholder="What needs to be done?"
+        onkeydown=${this.createTodo.bind(this)} />
+    </header>`
+  }
+
+  createTodo (e) {
+    var value = e.target.value
+    if (e.keyCode === 13) {
+      e.target.value = ''
+      this.emit('todos:create', value)
+    }
+  }
+}