Add ability to match part of a required extension

[dburkart]

For instance, the :comparator flag is governed by "comparator-*", i.e. to use syntax like:

if header :value "lt" :comparator "i;ascii-numeric"
            ["x-priority"] ["3"]
 {
     fileinto "Priority";
 }

One would need the following require statement:

require "comparator-i;ascii-numerif";

To support this, we need the ability to match part of a required extension.