Add flag for "dry-run" mode of evaluable comments

[TheMatten]

When using --allow-eval with unfamiliar codebases, it may be useful to have flag that disables execution of evaluable comments, while still showing them in REPL for inspection - compared to e.g. custom Setup.hs, these comments aren't contained in one, easy to inspect file, but can be scattered throughout the codebase, possibly including some malicious code that could be overlooked.

[ndmitchell]

Seems perfectly reasonable. Patch welcome!