Allow to exclude specific entries from the bibliography

[goerz]

From https://discourse.julialang.org/t/ann-documentercitations-v1-0-0/101562/6

I'm not exactly sure what the use case is for this, but it would certainly be easy to implement.

The most straightforward syntax for showing all references in the .bib file except for three specific ones would probably be

```@bibliography
*
! BrifNJP2010
! GoerzDiploma2010
! GoerzPhd2015
```

That is, !, followed by a space (since technically citation keys are allowed to start with !)