Feature request: extend from other config files

[PaulRBerg]

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I find it common for Echidna tests that belong to a specific contract to require a bespoke configuration file. One contract may only be called by an admin, while another one may need some functions whitelisted or blacklisted.

This is how I structure my Echidna invariants:

contracts
└── invariants
    ├── FooInvariants.sol
    ├── foo.yml
    ├── BarInvariants.sol
    └── bar.yml

A lot of fields get repeated across the *.yml files. It'd be nice if I could have "child" Echidna configs that extend from other "parent" Echidna configs, similar to how I can do this with Solhint.

[gustavo-grieco]

This is implemented in echidna-parade. More info is coming soon..