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

bin/utils/openapi-generator-cli.sh

@@ -0,0 +1,60 @@
+#!/usr/bin/env bash
+####
+# Save as openapi-generator-cli on your PATH. chmod u+x. Enjoy.
+#
+# This script will query github on every invocation to pull the latest released version
+# of openapi-generator.
+#
+# If you want repeatable executions, you can explicitly set a version via
+#    OPENAPI_GENERATOR_VERSION
+# e.g. (in Bash)
+#    export OPENAPI_GENERATOR_VERSION=3.1.0
+#    openapi-generator-cli.sh
+# or
+#    OPENAPI_GENERATOR_VERSION=3.1.0 openapi-generator-cli.sh
+#
+# This is also helpful, for example, if you want want to evaluate a SNAPSHOT version.
+#
+# NOTE: Jars are downloaded on demand from maven into the same directory as this script
+# for every 'latest' version pulled from github. Consider putting this under its own directory.
+####
+set -o pipefail
+
+for cmd in {mvn,python,curl}; do
+  if ! command -v ${cmd} > /dev/null; then
+  >&2 echo "This script requires '${cmd}' to be installed."
+    exit 1
+  fi
+done
+
+function latest.tag {
+  local uri="https://api.github.com/repos/${1}/tags"
+  curl -s ${uri} | python -c "import sys, json; print json.load(sys.stdin)[0]['name'][1:]"
+}
+
+ghrepo=openapitools/openapi-generator
+groupid=org.openapitools
+artifactid=openapi-generator-cli
+ver=${OPENAPI_GENERATOR_VERSION:-$(latest.tag $ghrepo)}
+
+jar=${artifactid}-${ver}.jar
+DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+
+if [ ! -f ${DIR}/${jar} ]; then
+  repo="central::default::https://repo1.maven.apache.org/maven2"
+  if [[ ${ver} =~ ^.*-SNAPSHOT$ ]]; then
+      repo="central::default::https://oss.sonatype.org/content/repositories/snapshots"
+  fi
+  mvn org.apache.maven.plugins:maven-dependency-plugin:2.9:get \
+    -DremoteRepositories=${repo} \
+    -Dartifact=${groupid}:${artifactid}:${ver} \
+    -Dtransitive=false \
+    -Ddest=${DIR}/${jar}
+fi
+
+java -ea                          \
+  ${JAVA_OPTS}                    \
+  -Xms512M                        \
+  -Xmx1024M                       \
+  -server                         \
+  -jar ${DIR}/${jar} "$@"