Add missing fields

[konsumlamm]

source-repository & flag sections now don't allow if conditions, there were missing fields for source-repository sections and the default value of default fields in flags is True.

I wrote separate parsers for source-repository & flag sections, since they share basically no fields with other sections and don't allow conditions. I'm open to suggestions on how to best reduce code duplication.

[augustss]

I don't see the harm in allowing if in more places than cabal does.
It's also the case that cabal does not allow if at the top level. I think this a flaw, since it's a useful thing.

[konsumlamm]

So should I just revert the parser changes and add the missing fields to parsers?

[augustss]

Yes, I think so. I'm not bothered by mcabal parsing some things that cabal doesn't. I hope mcabal will will be temporary and that eventually we can use cabal.