Add the possibility to simplify sequential citations

Thanks for the contributions!

Few comments:

1. We should at least warn that backlinks will no longer work (e.g. 1-5 won't link back to 2, 3, etc)
2. I believe we should use 'en dash' to match standards
3. Is the idea of preprocess hook because we don't have a single spot in the plugins/classes to build them? Wondering if maybe it's time to have a footnotes links builder service. Worried if we put the code to build them in too many places it'd be even harder to fix issues.
4. And finally this needs test coverage, too much going on in the footnotes backlinks already to add things into there without coverage

Thanks!

Is the idea of preprocess hook because we don't have a single spot in the plugins/classes to build them?

Yes, just to make it work anywhere #theme => footnote_links is used. I guess it's only used at one place in the module, but it could also be used in another module. And also to support controlling this behaviour by passing #simplify_sequential_citations => TRUE/FALSE.