Add duktape 1.5.1, an MIT-licensed Javascript engine.

This is the command-line tool, which uses the recently added libduktape package.

Duktape is an embeddable Ecmascript E5/E5.1 engine with a focus on
portability and compact footprint.  By integrating Duktape into your
C/C++ program you can easily extend its functionality through
scripting.  You can also build the main control flow of your program
in Ecmascript and use fast C code functions to do heavy lifting.

The terms Ecmascript and Javascript are often considered more or less
equivalent, although Javascript and its variants are technically just
one environment where the Ecmascript language is used.  The line
between the two is not very clear in practice:  even non-browser
Ecmascript environments often provide some browser-specific built-ins.
Duktape is no exception, and provides the commonly used print() and
alert() built-ins.  Even so, we use the term Ecmascript throughout to
refer to the language implemented by Duktape.

(agc)