Added support for class (function) mixins

Throw error when referencing an unregistered mixin

Before this, `this.mixin('nonexistent-mixin')` threw `Uncaught TypeError: Cannot convert undefin...

Added component

A 3d flippable comonent

Added Happy-CSS resource for Tutorials

#### Content
Just added a link to _probably_ valuable learning resources for beginners.

Add demo to README.md: react vs riot simple comment box

Fixes #1697

`value` was leaking out of update code, and getting set directly on tags. That's a problem if yo...

When an event triggers, we want to traverse the DOM upwards until we find a riot tag, then call ...

The compiler expects the closing tag to not have any leading whitespace. That's a bit awkward lo...

Writing and debugging tests is cumbersome because the tag html is usually in another file, requi...

#1659 brought up an issue where you'd have both `each` and `if` on a custom tag. In that case, t...

Added ability to have multiple global mixins

Unmount DOM when `if` becomes false

This lets you dynamically inject tags into the tree, which is especially useful for navigation.

It was inconsistent that update would only sometimes be run during a mount. This caused tags to ...

It sometimes happens that you'll call update after a tag has been removed (xhr, timer, etc). Onc...

When the `if` is initially false, then each will miss the before-mount event. Instead, we need t...

The main goal here was to remove references from the parent when an element gets removed because...

This simplifies code by not needing a TagRef object between the parsing and update phases. It al...

`each` has an internal array that stores the tag instance of every item being rendered. To make ...

The reason is that now when an if-attribute becomes false, it unmounts just that part of the DOM...

The code for setting (and removing) an object on a specified key was duplicated. This refactorin...

Before this change, we'd keep the old DOM, and simply avoid updating if when the if-attribute wa...