Add completions for caddy

[adriaanzon]

Description

Added completions for the various options of Caddy:

Usage of caddy:
  -agree
    	Agree to the CA's Subscriber Agreement
  -ca string
    	URL to certificate authority's ACME server directory (default "https://acme-v01.api.letsencrypt.org/directory")
  -catimeout duration
    	Default ACME CA HTTP timeout (default 10s)
  -conf string
    	Caddyfile to load (default "Caddyfile")
  -cpu string
    	CPU cap (default "100%")
  -disable-http-challenge
    	Disable the ACME HTTP challenge
  -disable-tls-sni-challenge
    	Disable the ACME TLS-SNI challenge
  -email string
    	Default ACME CA account email address
  -grace duration
    	Maximum duration of graceful shutdown (default 5s)
  -host string
    	Default host
  -http-port string
    	Default port to use for HTTP (default "80")
  -http2
    	Use HTTP/2 (default true)
  -https-port string
    	Default port to use for HTTPS (default "443")
  -log string
    	Process log file
  -pidfile string
    	Path to write pid file
  -plugins
    	List installed plugins
  -port string
    	Default port (default "2015")
  -quic
    	Use experimental QUIC
  -quiet
    	Quiet mode (no initialization output)
  -revoke string
    	Hostname for which to revoke the certificate
  -root string
    	Root path of default site (default ".")
  -type string
    	Type of server to run (default "http")
  -validate
    	Parse the Caddyfile but do not start the server
  -version
    	Show version

see https://caddyserver.com and https://github.com/mholt/caddy

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.md

[faho]

Merged, thanks!