Allow temporary exceptance of other certificate authorities

Hi,

I would really like to be allowed to add CACert as a CA for a TBB session (process would be accessing the certificates either via https://cacert.org or local files, clicking on them and checking the "Trust this CA to identify websites.". Because I rather carry the fingerprint of this one CA than all the CACert certified websites I might use.

This also plays into Tails where I might want to trust CACert persistently.

Affected versions: Tor Browser in Tails 1.2.2 TBB 4.0.2 x64 TBB 4.5-alpha-2 x64

Thank you.