feature: allow sites to be disabled through file naming

[khawkins98]

A small change that allows a site file configuration to be disabled by ending it with .disabled.js.

I've found this easier/preferable to temporarily deleting the file.

Another approach would be to add a enabled: true/false property, but this was simpler to implement -- and I think it's more obvious if a site is disabled.

Maybe you'll like the idea.

Update

I've now seen it's possible to add a skip: true parameter.

I think that makes this PR not required -- however I think this could have been more obvious, open to a PR adding that to the readme?