Add a helper utility

[one-d-wide]

Add small portable cli utility allowing to individually manipulate simple hosts.d entries. A quick interface showcase from the readme:

$ hosts.d -h # --help
A front-end to individually manipulate simple hosts.d entries

Usage: ./hosts.sh [OPERATION] [OPTIONS] [--] [ARGS]
Operations:
  -c, --create [HOST] [ADDR]
  -m, --move [SRC_HOST] [DST_HOST]
  -d, --delete [HOST]
  -r, --resolve [HOST]
  -l, --list [GLOB]
Options:
  -t, --target-dir <path> [default: '/etc/hosts.d']
  -p, --priority <priority>
  -f, --force
  -q, --quiet
      --resolver-script <script>
      --resolver <command> [default: 'dig']
      --resolver-arg <arg> [default: '+short']
      --resolver-erase-args
      --dry-run

$ hosts.d localhost 127.0.0.1 # -c, --create
Writing '127.0.0.1 localhost' to '/etc/hosts.d/localhost.conf'

$ hosts.d -l # --list
## From /etc/hosts.d/localhost.conf ##
127.0.0.1 localhost

$ hosts.d -m localhost local # --move
Writing '127.0.0.1 local' to '/etc/hosts.d/local.conf'
Deleting '/etc/hosts.d/localhost.conf'

$ hosts.d -m local -p 99 # --priority 99
Moving '/etc/hosts.d/local.conf' to '/etc/hosts.d/99-local.conf'

$ hosts.d -d local # --delete
Deleting '/etc/hosts.d/99-local.conf'

$ ./hosts.sh -r google.com # --resolve
Executing: 'dig' '+short' 'google.com'
Writing 'XX.XX.XX.XX google.com' to '/etc/hosts.d/google.com.conf'