wrapper for the builtin Perl interpolation engine

 String::Interpolate provides a neat interface to the solution to that
 perenial Perl problem - how to invoke the Perl string interpolation engine on
 a string contained in a scalar variable.
 .
 A String::Interpolate object encapsulates a string and a context in which it
 should be subjected to Perl interpolation. In the simplest, default, case the
 context is simply the namespace (package) from which the constructor was
 called.