Support for $1, $2, etc

[mightybyte]

Any chance you would be open to modifying this package to support $1, $2, etc? Those kinds of substitutions are useful when you have an array of "parameters" to be substituted instead of key/value pairs.

[tibbe]

I'm not sure how to square support for that with the way contexts are currently defined. If you have an idea I'm all ears.

[mightybyte]

I was thinking of something simple like this.

#11