Extend gmail normalizer

[ptman]

Now it only covers gmail.com and googlemail.com. Maybe refactor it as a separate thing from general email normalization and make it possible to pass in a slice or map[string]bool of domains that should receive gmail handling. Or check if MX records point to gmail email servers.

[sergeyglazyrindev]

Hello guys! I forked this package cause owner disappeared. Hope, he will be back, but it would be easier to merge these changes back if he is back Link to my repo: create issue there and we'll discuss it.