Adding preferred domains

[Dynom]

Certain e-mail providers discriminate based on their TLD. Hotmail, for example, can have e-mail addresses on Hotmail.fr or Hotmail.nl which won't work on Hotmail.com. While all domains are legitimate e-mail receiving domains.

To compensate for this behaviour, I've added the feature to have a preferred mapping. This PR introduces the mapping, which you can specify in the configuration file or as runtime flags. This allows you to hint your users to a more preferred domain, while keeping the option open to allow for their specific choice.

The preferred alternative will be prepended to the otherwise found alternative. This means it'll also work on bad (but syntactically valid) results.

Example

This will offer the following suggestion:
Request

{
    "email": "this-is-just-a-test@hotmail.ln"
}

Response

{
    "alternatives": [
        "this-is-just-a-test@hotmail.com",
        "this-is-just-a-test@hotmail.nl"
    ],
    "malformed_syntax": false
}

Configuration

config.toml

     [services.suggest.prefer]
        # The syntax is: "<domain>" = "<preferred domain>"
        "hotmail.fr" = "hotmail.com"
        "hotmail.nl" = "hotmail.com"

flags
The equivalent with flags:

./web --services-suggest-prefer hotmail.fr=hotmail.com --services-suggest-prefer hotmail.nl=hotmail.com

[codecov-commenter]

Codecov Report

Merging #16 into master will increase coverage by 0.10%.
The diff coverage is 63.15%.

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
+ Coverage   47.76%   47.87%   +0.10%     
==========================================
  Files          35       36       +1     
  Lines        1790     1832      +42     
==========================================
+ Hits          855      877      +22     
- Misses        917      933      +16     
- Partials       18       22       +4     

Impacted Files	Coverage Δ
cmd/web/main.go	0.00% <0.00%> (ø)
cmd/web/config/config.go	75.26% <17.64%> (-12.89%)	⬇️
cmd/web/services/suggest.go	88.70% <83.87%> (-6.85%)	⬇️
cmd/web/preferrer/preferrer.go	100.00% <100.00%> (ø)

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