582f1fe4

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Add `ignore` config option to skip mutating selected calls

The plugin now reads an `ignore` list from the YAML config (`--config=PATH`).
An expression whose syntactic head matches any name on the list is left
untouched, and so are all of its sub-expressions. Use this to silence noisy
mutations on calls whose removal or modification does not affect observable
behaviour, e.g.

    ignore:
      - logDebug
      - logInfo

Without this, the `RemoveAction` operator turns
`do { logDebug "x"; doWork }` into `do { doWork }`, which no test will fail
on, producing surviving mutants that are pure noise.

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