Add doctest pkg to octave-forge

[Colin Macdonald]

Hi Carnë, others,

Michael Walter and I, with generous help from Oliver Heimlich, have been 
polishing up our Doctest package for running tests on @example blocks in 
m-file documentation.

Could we add this to Octave-Forge?  Its being used by Oliver's Interval 
and my Symbolic pkgs.  Its maintained in git.

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If anyone wants to try it, 
[download](https://github.com/catch22/octave-doctest/releases) and then do:

>> pkg install doctest-0.4.0.tar.gz
>> pkg load doctest
>> doctest doctest    % test itself

Thanks,
Colin

--
Colin Macdonald
Associate Professor
Tutorial Fellow at Oriel College
University of Oxford