Support `kbd`, `btn`, and `menu` macros in AsciiDoc

[michaelblyons]

Please add support for rendering more AsciiDoc macros.

1. kbd:[Ctrl+P] => <kbd>Ctrl</kbd>+<kbd>P</kbd>
2. btn:[Okay] => <kbd>Okay</kbd>, <code>Okay</code>, <strong>Okay</strong>, "Okay", or even just Okay.
3. menu:File[Open recent > foo.txt] => <strong>File &gt; Open recent &gt; foo.txt</strong> or same but <em>.

Feel free to moot alternatives, but please do the processing!