Adding option "--no-del" to include only additions

[tdegeus]

Fixes #66

• The name of the option can potentially be improved.
• Seems to work well in the body.
• Does not yet work well in title (and potentially other parts of the preamble).
• I could not find how to really test. It seems that verify just checks that things compile.

[tdegeus]

@ftilmann It would be great if you could find some time to review this

[ftilmann]

Sorry for the long delay. I wanted to test with all the files in testsuite, but it passed without problems.

[tdegeus]

Thanks!