Consider load_remote_stylesheets default behavior

[ctidd]

Assuming the 80% use case for this crate is along the lines of email preprocessing, following links by default from within a service strikes me as an availability risk that the golden path would guide consumers away from, and allow them to opt in based on their specific use case.

It might be worth isolating load_remote_stylesheets to a crate feature and/or disabling by default, at least for remote URLs.

Edit: I noticed the http feature, so I'll leave this open as feedback regarding the defaults.