Allow to specify module re-exports in .ormolu files

[mrkkrp]

Close #1017.

It turns out that gaining knowledge of all module re-exports in the ecosystem is tricky because the Hoogle database does not contain this information, so one would need to download all 14+k packages from Hackage and parse their source code. We've decided that perhaps hardcoding some common re-exports and otherwise giving the users a way to make Ormolu aware of arbitrary important re-exports is a satisfactory solution for now.

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[mrkkrp]

@amesgen I've addressed your comments. Yeah, I think we can release 0.7.0.0 without support for mixins. This can be added later.