Automatic test

[javl]

A lot of extra functionally, far beyond my original need, has been added by contributors.
It would be nice to have some way of testing the code after updates to make sure all options still work.
One of the strengths of image2cpp in my opinion is the fact it's just a single .html page, so I'm not too keen on adding a whole test suite, but I'm also not sure what would be an easy way to add some testing functionality.