Add key macro to identify a key reference or key combination

[GoogleCodeExporter]

A very common need for computer-related documentation is to reference a key or 
key combination on a keyboard. These references are often styled in the output 
to appear as keys on a keyboard. Without proper semantics in an AsciiDoc 
document, it's not possible to identify this information in the output.

I propose adding an inline key macro that recognizes the following syntax:

key:[Enter]
key:[Ctrl+Shift+T]
key:[Ctrl+N]

I'm also considering using the target slot for a single key reference:

key:Enter[]

We could consider using the attributes for the modifiers and the target for the 
final key, as in:

key:N[Ctrl]

...however, I'm quite certain documentation writers won't find that intuitive.

In the docbook backend, each key character is mapped to <keycap> and multiple 
keys are wrapped in <keycombo>.

<keycap>Enter</keycap>
<keycombo><keycap>Ctrl</keycap><keycap>N</keycap></keycombo>

In the html backend, these element names become class names on <span> elements.

I've included an implementation and a page that shows examples in a gist:

https://gist.github.com/mojavelinux/4736691

Original issue reported on code.google.com by dan.j.allen on 8 Feb 2013 at 5:11