Add a linkchecker for Jekyll Site

[souravsingh]

This PR is related to Issue #62 for checking broken links.

[peterjc]

Good news - it works :)

Bad news - lots of broken links :(

Unless this is setup as a TravisCI build target which is allowed to fail, it will not be useful to commit this yet. Hopefully we can just fix the links...

You should probably tell linkchecker to only look at ./_site as per the example on #62 which would avoid the false warning from ./_layouts/default.html etc.

I just resolved the Gmane problem with bd236b7 and a malformed link this found with 97005af

Most of the errors are about the GitHub editing links, can you see what is going on there?

[peterjc]

Also as per the example on #62, using something like --only-4xx might be needed for all the (mostly valid) DOI links are getting Connection refused, and GitHub links returning HTTPTooManyRequests

[souravsingh]

@peterjc I will look into the issue with the false alarms.