Option to check external references

[hansspanjers]

Ape should be abel to check if all links are valid, even external links. Now, Ape does not check external links at all. It should be an option to do so.

Sometimes you don't want to let the outside world know what you are doing, so the default behavior should be "check internal links only".

[mthuurne]

Specifically, it would be useful to check the HTTP status code for external links and report for example Not Found, Moved Permanently and Permanent Redirect as problems. Checking the content for external links seems less useful.