This pull-request adds the ability for users to add a tag value to their feed, and use that in the email-template which is sent.

By default the tag isn't present, but if it is configured it will appear in the subject, after the [rss2email] marker:

  Subject: [rss2email] {{if .Tag}}{{.Tag}} {{end}}{{.Subject}}

The tag can be added like so:

$ cat ~/.rss2email/feeds.txt
https://rachelbythebay.com/w/atom.xml
  - Tag: people
https://blog.steve.fi/index.rss
  - Tag: people

There is only support for a single Tag: value in the configuration file, however if you wish you can set a value such as foo,bar and use the split function which is available within the template to append values & etc.