add support for multiple subaddress separators

[lewxdev]

According to Wikipedia, email providers may use different symbol delimiters before the subaddress part because of the lack of standardization. For example, according to the article, yahoo mail uses the hyphen (-) instead of the plus (+). Unfortunately, because I only skimmed the details initially, I missed this and never implemented any safeguards.

In order to support more email providers, this can be solved in one of a few ways:

1. Use plus (+) by default with some mapped exceptions based on the provider.
2. Use an extensive map of email providers to subaddress delimiters.

The second option may be better. This way, if the user has an email from a provider we haven't listed, we should assume the provider doesn't support subaddressing and prevent it from being added / prevent later issues.

[lewxdev]

will be resolved with #2