Add Homebrew formula for "hostess"

Add the formula for hostess (1), an idempotent command-line utility for
managing your `/etc/hosts` file.

References:
  (1) https://github.com/cbednarski/hostess

Resolves GH-174

snowflakes/homebrew/Brewfile

@@ -62,6 +62,7 @@ brew "git-sizer"
 brew "gnu-sed", args: ["with-default-names"]
 brew "gnu-tar", args: ["with-default-names"]
 brew "gnu-witch", args: ["with-default-names"]
+brew "hostess"
 brew "htop", args: ["with-ncurses"]
 brew "httpie"
 brew "hub"