Update unit files for systemd

Unlike network.target the network-online.target guarantees that the network
devices are online.

If you bind to 0.0.0.0, [::], [::1], and/or 127.0.0.1 only that is enough to
proceed. But in case a particular IP is needed, like ${COREOS_PUBLIC_IPV4},
we require any IP assignments to have completed before Caddy's start. That
is achieved by depending on systemd-networkd-wait-online.service (which is
scheduled before network-online.target, then, automatically).

Fix for #659.

systemd unit file

Add systemd service file for caddy

Add some README with basic setup instructions

Explain how to view the service configuration

Add a note about permissions

Add a comment about run user and group

service->service unit

A systemd service can consist of different units. A unit configuration
file has the `.service` file ending which is a bit confusing, so please
be considerate if I'm confusing `service` and `unit` in the README

Fix typos/reword

Add contact information

markdown: Included files in Markdown templates have access to document vars

Refactor how middleware.Context includes files

mime: Use a map, and error on duplicate extensions.

 - The mime middleware used filepath where it should arguably use path.
 - Changed the configuration to use a map instead of scanning an array
   during every request.  The map is static (after configuration), so
   should be fine for concurrent access.
 - Catch duplicate extensions within a configuration and error out.
 - Add tests for new error case.

proxy: Add max_conns parameter for per-host maximum connections

Play nice when ACME used manually.

Add flag to list directives.

Folder for init scripts within dist / FreeBSD service script

Hide only the currently used Caddyfile

Minor change to server/server.go such that /.well-known/acme-challenge
can be passed through when TLS.Manual is true on the vhost the request
came in through.

capitalize struct name in comment (go lint)

# Conflicts:
#	middleware/fastcgi/fastcgi.go

That was weird, only half of the file got committed...

Added a directory dist/init/ that may provide service scripts for various distributions in the future, added a experimental FreeBSD service script