Add redirect rule for biorxiv

[zafarali]

This PR adds support for biorxiv links that suffer from the same issue.

Tested on some links:

> rule = ["*://*biorxiv.org/*", /(.*)\/biorxiv\/(.*?)(.full.pdf)?$/, '$1/$2']
[ '*://*biorxiv.org/*',
  /(.*)\/biorxiv\/(.*?)(.full.pdf)?$/,
  '$1/$2' ]
> link = https://www.biorxiv.org/content/biorxiv/early/2017/09/18/113480.full.pdf
> link = 'https://www.biorxiv.org/content/biorxiv/early/2017/09/18/113480.full.pdf'
'https://www.biorxiv.org/content/biorxiv/early/2017/09/18/113480.full.pdf'
> link.replace(rule[1], rule[2])
'https://www.biorxiv.org/content/early/2017/09/18/113480'
> link = https://www.biorxiv.org/content/biorxiv/early/2018/03/03/275545.full.pdf
> link = 'https://www.biorxiv.org/content/biorxiv/early/2018/03/03/275545.full.pdf'
'https://www.biorxiv.org/content/biorxiv/early/2018/03/03/275545.full.pdf'
> link.replace(rule[1], rule[2])
'https://www.biorxiv.org/content/early/2018/03/03/275545'

If there are any sanity checks to run let me know!

[imurray]

Thanks for the pull request! I created a separate commit because I wanted to tweak it a bit. The first element of the rule needed an extra '.' to work in Firefox. I also tweaked the regex so links like https://www.biorxiv.org/content/early/2018/03/06/276774.full.pdf.