Check for redirecting pages if links.redirects is enabled

This is related to the feature request #20 I did some time ago.

I was not sure about the languages.yaml so I did put the english translation for the languages I didn't know.
The contribution docs tell me to write tests for any added functionality, but I didn't see any test infrastructure I could add something to.
Personally I would prefer to enable the redirects option by default, but since I was not sure I choose the option with the fewest possible impact.

to allow for the same code to enhance the links class and attribute I actually modified the method isExternalLink.
Since pages can redirect to other pages, I added a recursive call (circular redirects might cause the page containing a link to that "circular page" to crash with something like stackoverflow. But I think this is an edge case.
The change in the README.md regarding links.www to false is just reflecting the value in external_links.yaml. If you think this doesn't belong into this PR I cant change it back.

Sommerregen / grav-plugin-external-links