Publish add filter function

[PolynomialDivision]

Make add_filter public. If you want to add a filter you need to call add_filter with the corresponding filter list as an argument. This can be:

• input_filters
• output_filters
• redistribute_filters
• install_filters

This commit puts the add_filter and the filter lists into the header so filters can be added outside the configuration.c file.

[jech]

No idea, at first sight it looks good to me. I would suggest putting some print statements inside do_filter to check why your filter doesn't match.

[jech]

What about doing https://github.com/jech/babeld/pull/86 instead?

[PolynomialDivision]

What about doing #86 instead?

Awesome! :) I will update this PR.