[RFC] Add strict mode for secure command execution

This adds a new setting `MiniMagick.strict_mode` that enables some
sanity checks for `CommandBuilder` execution.

I did this because I'm afraid that passing some bad parameters to `CommandBuilder` could lead to arbitrary shell execution and I don't think the default flexibility of `CommandBuilder` is needed given its context.

Then again I'm not sure that strict mode fits all use cases, so I left it disabled by default.

I also should probably move some of the tests into a `ToolTest` suite, because that's where they belong.