Disable local link checking

[d0ugal]

As brought up in #146 it would be useful to disable the link checking particularly when migrating documentation from a previous system.

I think this would work best as a command line flag rather than a config option as we don't want to encourage using it long term.

[lovelydinosaur]

Perhaps we should simply warn for stuff like this by default and include a --strict option.

I think this would work best as a command line flag rather than a config option as we don't want to encourage using it long term.

Given that any config option can be specified as a command line flag we should prob just stick with that.

[d0ugal]

Fixed by #279