Add a warnings addendum file and a --mostly-hide-warnings switch

[andrewferrier]

• Warnings addendum file should be placed in same directory as PDF always if there are any warnings. It should be called _warnings.txt. Suggest we write another logging handler for this.
• Also, if there are warnings, the original email should also be placed in the same directory as the PDF, called, _original.eml.
• Warnings should also be output to stderr unless --mostly-hide-warnings is passed, in which case they shouldn't (and email2pdf shouldn't exit with a 1). Can be implemented by syserr_handler.setLevel(logging.ERROR), plus needs to exit with rc=0.