Option to warn about external resources

[mthuurne]

Many libraries for web applications load resources from CDNs by default. There are situations where this is undesirable, for privacy or robustness reasons. So it would be useful if APE could warn about it.

[mthuurne]

This could be combined with #8 perhaps, so the options for handling external resources would be:

• flag as an error
• ignore (like now)
• do a HEAD request and check status code

[mthuurne]

This feature would be more useful if #6 is implemented as well: resources such as fonts will often be linked from CSS rather than from HTML.

[mthuurne]

Perhaps instead of a single policy, different base URLs could be provided for each option. For example, check the existence of known external resources, flag everything else as an error.