Support "security.txt"

[tja]

Moving this issue from our internal issue tracker to Github as a request for this has come up:

Currently, Repo Lookout tries to find a contact email address by 1) checking the WHOIS record, 2) scraping the website for relevant emails (i.e. emails using a domain related to the hostname in question), 3) guessing email addresses if there's an MX record, and 4) extracting email addresses from the git repo itself.

This system should be extended to support the security.txt standard as well.