Mail merge plugin

Implement a plugin for basic mail merge functionality. This allows:

• Creating a template containing merge fields such as {{name}}
• Merging the template with a CSV file (UTF-8 encoded, quoted, comma separated, first row contains column names), where each row turned into an email, and each field is replaced with the value from the corresponding column
• Sending the merged email messages throttled so as to not hit common mail server limits

Currently, this only supports sending HTML+text mail, direct string substitutions (no conditional fields) and no templated attachments.

Fixes #660 (closed)