Adds a simple bash completion script (#277)

* Adds a simple bash completion script

This works with any loading script named openapi-generator-cli.
That is, if you've installed via homebrew or created a script similar
to https://gist.github.com/jimschubert/ce241b0c78140e364f46914ef8ec4103

This script is relatively simple, relying on fallback to the recently
add "completion" command to the CLI project.

The script includes a possible extension to allow for per-language
options to autocomplete when the user is applying additional properties.
This work is currently commented out, as it may be simplified a bit in
the CLI first.

* Add launcher script and "install" instructions

scripts/openapi-generator-cli-completion.bash

@@ -0,0 +1,46 @@
+#!/usr/bin/env bash
+
+###
+# Provides completion assistance for openapi-generator-cli
+# Install
+# Mac:
+#    brew install bash-completion
+#    cp openapi-generator-cli-completion.bash `brew --prefix`/etc/bash_completion.d
+# Linux: many distributions include this automatically. Search for your distro-specific instructions.
+# When in doubt, try sourcing this file:
+#    type complete && source openapi-generator-cli
+#
+# see http://tldp.org/LDP/abs/html/tabexpansion.html
+###
+
+_openapi_generator_cli_completions() {
+  COMPREPLY=()
+  local IFS=$' \t\n'
+  local options=()
+
+  options+=("$($1 completion ${COMP_WORDS[@]:1})")
+
+  case "${COMP_WORDS[1]}" in
+    generate)
+      case "${COMP_WORDS[@]:2}" in
+        -l|--lang|-g|--generator-name)
+        # TODO: This is a nice-to-have and not required.
+        # Apply generator-specific options to additional properties. These can be queried via:
+        # openapi-generator-cli config-help -l YOUR_LANG | grep '^\t' | grep -v '^\t\s\s\s\s' | tr -d '\t'
+        # where YOUR_LANG would need to be evaluated as the value after the current switch.
+        # but rather than switching on 'generate' maybe switch on --additional-properties?
+        ;;
+      esac
+    ;;
+    *)
+    # ignore
+    ;;
+  esac
+
+  # printf '%s\n' "${options[@]}"
+  if [[ -n "${options[@]}" ]]; then
+    COMPREPLY=( $(compgen  -W "${options}" -- ${2}) )
+  fi
+}
+
+complete -F _openapi_generator_cli_completions openapi-generator-cli