Extension points in `org.dita.htmlhelp`

Certain extension points are specific to the HTML Help transformation.

dita.xsl.htmlhelp.map2hhc: Overrides the default XSLT step that generates the HTML Help contents (.hhc) file.
dita.xsl.htmlhelp.map2hhp: Overrides the default XSLT step that generates the HTML Help project (.hhp) file.

`t`	Open the topic finder to filter the list of topic titles
`s`	Use the search field to find text in topic content
`?`	Show this list of keyboard shortcuts

Extension points in org.dita.htmlhelp