Add --dump-to option to save the messages to the filesystem

[nickolay]

This adds a --dump-to option, which can be:

• either a path to an existing directory (in which N.eml files will be created)
• or a jinja template specifying the file names to create (e.g. ./{{_mailmerge_message_num}}.eml)

The feature was requested in #103

There are no tests and I didn't check anything more complex than running pytest on Python 3.8, but once the approach is agreed on I can go through the instructions.
(Python 2 is EOL, do you still want to support it?)

[codecov-io]

Codecov Report

Merging #107 into develop will decrease coverage by 2.10%.
The diff coverage is 61.11%.

@@             Coverage Diff             @@
##           develop     #107      +/-   ##
===========================================
- Coverage    99.03%   96.93%   -2.11%     
===========================================
  Files            6        6              
  Lines          312      326      +14     
===========================================
+ Hits           309      316       +7     
- Misses           3       10       +7     

Impacted Files	Coverage Δ
mailmerge/__main__.py	94.92% <50.00%> (-4.30%)	⬇️
mailmerge/template_message.py	99.09% <83.33%> (-0.91%)	⬇️

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[awdeorio]

This is a great sketch of a first step towards implement reliable email delivery #36. I'm going to close in favor of a more comprehensive future solution that include reliable delivery.

As per the discussion in #103, using mailmerge as a message generator only can be accomplished by mailmerge --output-format raw together with a short script. You could also use csplit, here's an example that could still use a bit of cleanup.

$ mailmerge --sample
$ mailmerge --no-limit --output-format raw | csplit - '/^>>> message.*sent*$/'
$ cat xx00
>>> message 1
TO: [email protected]
SUBJECT: Testing mailmerge
FROM: My Self <[email protected]>
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Date: Mon, 02 Nov 2020 20:25:38 -0000

Hi, Myself,

Your number is 17.
$ cat xx01
>>> message 1 sent
>>> message 2
TO: [email protected]
SUBJECT: Testing mailmerge
FROM: My Self <[email protected]>
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Date: Mon, 02 Nov 2020 20:25:38 -0000

Hi, Bob,

Your number is 42.
>>> message 2 sent
>>> This was a dry run.  To send messages, use the --no-dry-run option.

[nickolay]

Didn't know about csplit, thanks!

I'm not sure I like the idea of gleaning the e-mails from the output intended for humans, though. Can the --output-format=raw be changed to print \0-delimited emails, without additional logging?

[awdeorio]

Maybe a --silent option, which would suppress all the >>> lines. Then you could just do

$ mailmerge --no-limit --output-format raw | csplit - '/^TO: /' '{*}'