Implemented custom types (fixes #33).

A custom type is specified by preceeding a class name with a colon, eg: `:Foo\Bar`. The parser produces an `ExtensionType` instance with the class name set to `Foo\Bar`.

Attributes are supported, and are passed to the `ExtensionType` instance unmodified, eg: `:Foo\Bar { myCustomAttribute: true }`.
Support for arbitrary attributes required modification of `Parser::parseAttributes()`, the `$supportedAttributes` parameter is now nullable to indicate that there is no restriction on the supported attribute names.

`ExtensionType` exposes two methods:
 * `className()` which returns a ClassName instance; and
 * `attributes()` which returns the attributes array.