Provide a normalisation function to match email addresses

[jace]

The following email addresses all point to the same mailbox:

1. example@gmail.com
2. example@googlemail.com
3. example+extra@gmail.com
4. exam.ple@gmail.com

There are use cases for "normalising" email addresses to confirm two different strings point to the same mailbox (see hasgeek/lastuser#221 for example). mxsniff should provide a normalisation function that:

1. Lowercases the domain portion. (The mailbox portion's case must be preserved as per RFC 5321. Lowercasing it for the purpose of indexing should be a downstream concern.)
2. Strips + suffixes from the mailbox portion.

A custom normalisation function for the google-gmail provider does additional processing:

1. Replaces @googlemail.com with @gmail.com
2. Strips . (periods) from the address

mxsniff will default to the standard normalisation function if a provider doesn't specify a custom function. Combined with #5, this can be used to normalise an email address without a DNS lookup, using just the domain to guess.