arxiv -> ar5iv

With this extension you will automatically see the arxiv pdf rendered as a (html5+mathJax) web-page, since it replaces arxiv links with ar5iv. This behaviour can be toggled by clicking the extension, it either always loads the html version or the pdf version. For more details of this FOSS extension see https://github.com/HannesGitH/arxiv--ar5iv#readme