Allow additive attributes

[kKronstainBrown]

Problem

Currently, multiple extensions that apply the same attribute can't be used together.

For example:

{:java: .ph data-hd-programlang='java'}
{:node: .ph data-hd-programlang='node'}

Because these both use data-hd-programlang, they can't both be applied to a single element, for example:

This is a paragraph.
{: java}
{: node}

Solution

Change how attributes are applied so that the Markdown parser adds the two values so that both are applied, for example data-hd-programlang='java node'