Fixing autocomplete and adding citation extraction

[dhufe]

Due to some additional quotes the code that updates the cached citation keys did not work anymore (bibfile_modifed and load_bibfile).

Additionally, I added some code to extract the used citation entries. This is dne using some GPL code provided by Joseph Reagle.

[mangecoeur]

Hi thanks for the PR, I just had a look and I left a comment - there is a case of joining paths that I think will fail on windows. Could you please check the comment and resolve thanks :)

[dhufe]

Sry for the late response. I will check it.