Add option to suppress text file illo text

[windymilla]

To support when PPer copies alt text from HTML
to use as text in [Illustration:...] markup in text file.

Fixes #1932

[windymilla]

Testing notes:

1. Use an HTML file containing an image that has alt text but no caption, and PPcomp it with the text file.
2. Ensure the text file just says [Illustration] (i.e. no caption)
3. This should not cause a diff
4. Now copy the alt text into the text file: [Illustration: Copied alt text]
5. In master, and in this branch by default, this should cause a diff, reporting that the text file has text that the HTML does not.
6. Turn on the new option, and the diff should go away.

I'm also hoping for a test file from the originator of the request to be certain it addresses the issue they raised.