Allow multiple wordlists via command line parameter

[grhm]

I have a sphinx build that is embedded in a larger build process and adding/editing the conf.py that is generated is non-trivial.
We have been adding our custom word list via a -D spelling_word_list_filename=my_words.txt parameter on the sphinx command line.

The documentation says you can use multiple text files with words to be added to the dictionary, if you set the spelling_word_list_filename to a list. This works if you set the list in the conf.py file, but using a -D parameter only allows the addition of a single word list. We'd like to add several, but the way that command line parameter is handled, it is always given to sphinxcontrib-spelling as a string.

It would be nice if I could add a list of filenames via -D spelling_word_list_filename=[my_words.txt,other_words.txt] or some similar format.

Looking at the code, I think a small change in builder.py to check the incoming string and, if it starts and ends with square brackets, split it into a list would work.
i.e. just before Line 109 (https://github.com/sphinx-contrib/spelling/blob/master/sphinxcontrib/spelling/builder.py#L109).

Would you consider adding this? If so, I can probably contribute a pull request to add it.

[dhellmann]

This work was released in 7.5.0