Make dump() more robust

[kalimag]

This PR modifies dump() to escape string values and make key/value handling consistent.

Main differences are:

• strings may contain double quotes, backslashes and new lines
• table keys may be booleans and tables
• other types that can't be sensibly serialized throw an error instead of getting stringified (this is subjective, but I think this would only ever happen due to new bugs)

This should not make any practical difference for the shuffler as-is, because all table keys are known strings and all string values currently found in the config are limited by rules for file names. It would only be relevant to future features and external plugins.

[authorblues]

I like it. I hadn't considered that plugins would really demand a bit more scrutiny from my dump() method.