Add support for multiple bibliography files

This PR adds support to use multiple instances of --file when using the bibliography tag in order to support multiple bibliographies, for example in the following way:

`{% bibliography --file journal-abbrev.bib --file pubs.bib %}`

To give an idea of why this is useful, I currently use three bibliography files: `pubs.bib`, `journal-abbrev.bib` and `journal-full.bib`. The `journal*.bib` files contain `@STRING` entires for abbreviated and full journal names respectively which are used in `pubs.bib`, so that depending on if I want full vs. abbreviated names I can just switch the filename in my LaTeX file.

There are a number of things to bear in mind with this PR:

1. I have never coded any Ruby before yesterday;
2. Possibly the file concatenation part should be in `BibTeX.open` method of `bibtex-ruby`;
3. There is no cucumber test for this since every effort I've made to get it working, even on vanilla master, has failed (it strikes me that this is probably a bug, but my inexperience with Ruby means I'm not sure!)
4. Looking at the code this probably breaks `jekyll-scholar-extras`.

This PR probably needs some refinement therefore but if there's interest I can tidy it up. 

Anyway, this is a cracking bit of software and thanks for releasing it. It's made my life a lot easier in trying to sanely manage my website publication lists!