[cleaner] clean mail addresses (e.g. from maillog)

[pmoravec]

sos clean should obfuscate mail addresses (esp. from maillog). Basically anything in form user@domain.tld should be treaten as a mail address to scrub.

We should have either a dedicated Prepper/Parser/Mapper, or (rather?) extend Username and Hostname cleaners to cover that - to align mapping among them (i.e. mail user007@domain.tld obfuscated to obfuscateduser1@obfuscateddomain2.tld should mean user007 is always mapped to obfuscateduser1 and domain.tld domain is always mapped to obfuscateddomain2.tld).