Adds option to use a Shibboleth SP discovery feed for metadata filtering

[klelifo]

Hi!
First of all: Great work on this tool and thanks for all the work you put into it!

And now: I did exactly what I was told not to do, cloned the repository and hosted my own DiscoJuice instance :) (at least in parts)

With this pull-request I added support to filter the great number of IdPs coming from one or more of the involved federations down to ones that are actually granted access to a certain Shibboleth SP.

As far as I understand it, the Shibboleth SP provides a special discovery feed URL that gives you a list of all IdPs that are known to the SP.
With this list I filtered the federation metadata, giving me a nice small list of IdPs that are allowed to use my SP.

Example is up and running at https://fauboxtest.rrze.uni-erlangen.de
Click on "SSO Anmeldung" to see the DiscoJuice overlay with the filtered list of IdPs.
See https://fauboxtest.rrze.uni-erlangen.de/Shibboleth.sso/DiscoFeed for the generated Shibboleth SP discovery feed.

I hope this is helpful to you.
Thx and cu

[klelifo]

I also added a fix for #42