* Can add allowed custom headers to an email Message. WIP.
* adds slug as a custom email header to all outgoing emails
* Headers are static - declared as key/value pairs in config. All static headers get added to emails.
* updates comment
* adds tests for parsing smtp static headers
* updates test to assert static headers are included when building email
* updates test to use multiple static headers
* updates test names
* fixes linting issue with error
* ignore gocyclo for loading config
* updates email headers in tests to be formatted properly
* add static headers first
* updates tests to assert that regular headers like From cant be overwritten
* ensures only the header is in a valid format for smtp and not the value
* updates comment and error message wording
* adds to docs and ini sample files
* updates smtp.static_headers docs examples formatting
* removes lines commented with semi colons
* prettier:write
* renames var
* Add abstraction layer above SMTP communication
* Fix issues with attachments and sync command
* Tests for bad SMTP behavior
* Separate tests between async and sync entry points. Test difference between them
* Return interface so Wire can properly map types
* Address feedback from George
