Genemail

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* Removed `distribute` dependency
* Improved email regex and made it globally pre-compiled

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* Added `cache` template parameter for per-email-instance value caching

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* Added override of "Message-ID" header to be based on `mailfrom`
domain (overrideable by setting Email.setHeader or generating in
template)

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* Added "genemail_format" rendering parameter

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* First tagged release
* Added encryption modifier (using GPG)
* Added defaulting of outbound "Date" header
* Make DKIM modifier support optional