Give proactive information to help fix issues

[nikitavoloboev]

Perhaps it would be good if it suggested if the broken link was broken because the file was moved somewhere. And if tool finds the file (same name but different location perhaps) it writes that out as part of the result. I think it would be nice to not have that be default behavior and have it be under a flag as often the user knows himself where the issues are.

Ideally the tool would even proactively suggest to change the links it can 'fix' for the user and have user accept or discard the fixes.

[ctm]

I'll add this feature when I also add support for checking fragments, because I am going to introduce a HashMap that knows about files. I had planned on canonicalizing the file paths and using that as a key, but since there won't be that many duplicated filenames, I can just use the filename as a key and then a Vec of the additional information needed (both for fragments and for suggestions) on a per-file basis.

I'll add line numbers first though, 'cause that should be easier.