Using protoc cli parameter to specify additional class derivations

[tim2CF]

The main feature of this PR is support of additional class instances for all generated types. Parameters for additional class deriving could be passed through protoc command-line argument. Example which enables stock deriving of GHC.Generics.Generic class for all generated haskell types:

--haskell_opt='Opt{ imports = [], pragmas = ["DeriveGeneric"], stockInstances = ["GHC.Generics.Generic"], defaultInstances = [] }'

• Introduce Data.ProtoLens.Compiler.Parameter module which enables usage of cli parameter from plugin.proto.
• Using Data.ProtoLens.Compiler.Parameter to enable additional stock deriving (useful for classes like GHC.Generics.Generic).
• Using Data.ProtoLens.Compiler.Parameter to enable additional default instances implementations (useful for classes which do support default implementations through GHC.Generics.Generic).

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[tim2CF]

This is quite small pull request, maybe I could receive some human feedback before google-cla bot approval. If changes could be merged in principle (maybe after some improvements) - then I'll sign cla. Otherwise it does not make much sense.

[blackgnezdo]

Thanks for the PR. It looks like a useful capability. I pointed out some areas for improvement. In addition to those, please add a test covering this newly added functionality.