Markbind: Support custom keywords

[jamos-tay]

Extension of #428

With the current implementation of keywords, users are forced to include these keywords in the text if they want them to be tagged to a heading.

Allow the option of having keywords invisibly tagged to a heading, for example:

# Using a Java IDE
<span class="keyword hidden">Eclipse</span>
<span class="keyword hidden">Netbeans</span>
<span class="keyword hidden">IntelliJ</span>

[yamgent]

Maybe this can just be a custom css style?

```
.keyword.hidden {
display: none;
}
````

[damithc]

using one class is better than two.

[acjh]

The case for Atomic CSS: http://johnpolacek.com/rethinking/

Since we already ship with Bootstrap 4, this should already work:

<span class="keyword d-none">Eclipse</span>

[damithc]

Since we already ship with Bootstrap 4, this should already work:

In that case, the first stage could be simply documenting this technique. At a later stage, we can introduce a simpler syntax.