store/libressl: initial support

[adamdecaf]

An extension of openssl support (#31), but the paths/commands are a bit different. This needs to work with openssl (not a new store type), but would take precedence over openssl paths/commands.

$ brew info libressl
libressl: stable 2.6.4 (bottled), HEAD [keg-only]
Version of the SSL/TLS protocol forked from OpenSSL
https://www.libressl.org/
Not installed
From: https://github.com/Homebrew/homebrew-core/blob/master/Formula/libressl.rb
==> Options
--HEAD
	Install HEAD version
==> Caveats
A CA file has been bootstrapped using certificates from the SystemRoots
keychain. To add additional certificates (e.g. the certificates added in
the System keychain), place .pem files in
  /usr/local/etc/libressl/certs

and run
  /usr/local/opt/libressl/bin/openssl certhash /usr/local/etc/libressl/certs

This formula is keg-only, which means it was not symlinked into /usr/local,
because LibreSSL is not linked to prevent conflict with the system OpenSSL.