Fork of cmp-bibtex with some additional functionality

I know many of you are shifting to blink, but I haven't quite gotten there myself yet. I wanted better bibtex-support in cmp, and found liamvdvyver/cmp-bibtex. It was a good start, but wanted some additional functionality. I've submitted a PR with the following improvements:

• more information in preview of bib-entry;
• formatted preview according to apa (could be set according to user's preferences in the future, reading settings);
• add entry type to preview;
• handle more special characters (umlauts etc) and some commands that might be found in titles and author names;
• support additional citation commands (biblatex: footcite, footcites, parencite etc.) as well as page references "\cite[9]{Author2020}" and "\cite[cf.][9]{Author2020}".

Haven't heard any response to the PR yet; until I do, I'll leave my fork public in case it would be of use to someone.