add citationkey completion from sources not listed in YAML header

[Konfekt]

The vim-pandoc-completion-module of https://github.com/vim-pandoc/vim-pandoc offers in addition to completion of citation keys of the BIB file given in the YAML header, that of keys in other sources, as listed in the variable g:pandoc#biblio#sources:

g:pandoc#biblio#sources
  default = "bcgy"

  Where to look for bibliographies:

    b: use files with the same name as the current file, in the working dir
    c: use any bibliography in the working dir
    l: look in pandoc's local dir
    t: look in the local texmf tree
    g: append items in |g:pandoc#biblio#bibs|
    y: look in the current file's YAML header

If this functionality could be merged in?

[patashish704]

Sure, give me a couple of days.