Fix #78: Email check settings

[mwarkentin]

• Add WATCHMAN_EMAIL_HEADERS setting
• Add WATCHMAN_EMAIL_RECIPIENTS setting
• Merge headers before sending email

TODO

• Tests
• Update docs

[mwarkentin]

By analyzing the blame information on this pull request, we identified @benwebber, @kezabelle and @none to be potential reviewers.

[dlanger]

👍

[coveralls]

Coverage decreased (-0.3%) to 82.906% when pulling 194fc59 on 78-custom-email-headers into d8bc755 on master.

[coveralls]

Coverage increased (+1.9%) to 85.043% when pulling 53625f0 on 78-custom-email-headers into d8bc755 on master.

[coveralls]

Coverage increased (+1.9%) to 85.106% when pulling 7cc230c on 78-custom-email-headers into d8bc755 on master.

[dlanger]

🚀

[coveralls]

Coverage increased (+1.9%) to 85.106% when pulling ad927f6 on 78-custom-email-headers into d8bc755 on master.

[coveralls]

Coverage increased (+1.9%) to 85.106% when pulling fdc2b0f on 78-custom-email-headers into d8bc755 on master.

[coveralls]

Coverage increased (+1.9%) to 85.106% when pulling fdc2b0f on 78-custom-email-headers into d8bc755 on master.