Fixed relative link checking for local input files.

[chrispomeroyhale]

The link checker will correctly evaluate a relative URL contained within markdown when a remote URL is specified as a CLI argument that contains a relative link. But it failed when a local filename was specified instead. This patch addresses that issue by considering whether the markdown source URL is local.

[tcort]

I'd rather do this in a more generic way. For example, in the markdown-link-check script, if it's a file being supplied, opts.baseUrl would get set to file:///your/path/here, and link-check would look at the scheme (http, https, file, etc) and choose the right checker (e.g. http/https would use request while file would use fs.stat). There would probably have to be a new command line option (for specifying the baseUrl for data from stdin): cat /path/to/*.md | markdown-link-check --base-url file:///path/to

[sethreidnz]

Want some one to try to implement this? I'm pretty keen because I was thinking about building literally that piece of what you have done here... Then I came across this repo and this thread... If you haven't already begun then I'll fork and give it a go.

[tcort]

I've added support for the file: protocol (version 3.0.0), so what I have above should work now.