[MlMt] Proposal: conditional image loading by URL filtering

[Sam Hathaway]

Hi Benny and friends,

I’ve been thinking about whether there’d be an easy way for you to 
add conditional domain-based image loading, without doing a bunch of UI 
work for a feature that (admittedly) has limited appeal.

How about this:

1. define a new advanced preference: `defaults write com.freron.MailMate 
MmExternalUriApprover -string /path/to/uriapprover`
2. If this is set, MailMate would run this program when displaying an 
HTML email. It would send the program each image URL on stdin, and 
expect to get back the strings `allow` or `deny`.
3. MailMate would use this information to decide whether to display each 
external image.

I would write my own `uriapprover` script. A simple version might look 
like this:

```perl
#!/usr/bin/env perl

use Modern::Perl;
use Text::Glob 'glob_to_regex_string';
use URI;

my @allowed = qw(
     *.slack.com
     *.amazon.com
     *.postmark.com
);

my $allowed = join('|', map(glob_to_regex_string($_), @allowed));
sub allowed { eval { return URI->new($_)->host =~ m/($allowed)/o } }
while (<>) { say allowed($_) ? "allow" : "deny" }
```

Everyone else could ignore this feature.

What do you think?
-sam
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