Support populating `%s` specifiers with capture groups

[jdkato]

Originally posted by @jdkato in #609 (reply in thread)

[daobrien]

This would be a great improvement. We'd take advantage of this immediately. Thanks

[jvoisin]

I wanted to take advantage of this to correct things like le avion → l'avion, le ane → l'âne in French :<

[jdkato]

I'm going to try to get this into the next release.