rustfmt: Handle all the possible values without introducting security issues

[szabgab]

• Currently we don't handle well values that are lists with []
• As each formatting option and each value will have a file listing the crates we have to make sure these are all valid filenames. (so for example they don't contain .. that would put the file in a folder outside of where it should be. We might need to use some escaping that will be good for both URLs and filenames.