added conditional extension delimiter

[calebanderson]

Characters wrapped in %d{} only appear when the extension is present. This would be able to be merged in now without changing any of the old behavior and allow an amount of flexibility that is quite useful.
I completely understand if this isn't in the spirit of the gem and doesn't get merged, but I found it useful and thought you might like the idea.

[elskwid]

@calebanderson I dig the idea. Did you give any thought to using a token related to %x? Like maybe %X? Wondering if we could make it easier for a person looking at the string to understand those things are linked.

[calebanderson]

@elskwid I did originally, but that ended up looking like %a %n%X{ x}%x and that had too many x's for my taste. It's a simple change though.
EDIT: I chose %d for extension delimiter.