Replace hard tabs that snuck in with ea04c37
Add support for only inspecting headers, not the body

Default is to still look in body, but if you want the module to only
look in the defined header, use Set($CommandByMailOnlyHeaders, 1);

(Based on commit 2b533dc27f6a074b96a39c66379fc150a3f51ede from pr/3.)

[Adjusted tabs — s/\t/    /g — and simplified the $only_headers
conditional. -trs]
Releng 0.13