[apache] Add `includeSubDomains` directive to HSTS

The `includeSubDomains` optional directive allows the user to apply the
`HTTP Strict Transport Security` rule to all of the site's subdomains:
* http://tools.ietf.org/html/draft-ietf-websec-strict-transport-sec-14#section-6.1
* https://developer.mozilla.org/en-US/docs/Security/HTTP_Strict_Transport_Security

Ref: h5bp/server-configs#151