Add `rbenv install --skip-existing <VERSION>` option

The `-f` option allows for forcing installation of versions that already
exist in the environment. If that option isn't used, then an interactive
prompt is created which causes problems for non-interactive execution of
the command.

This change adds the `-s/--skip-existing` option to not install versions
that already exist, while still exiting as success.

Closes #543

bin/rbenv-install

@@ -8,6 +8,7 @@
 #
 #   -l/--list          List all available versions
 #   -f/--force         Install even if the version appears to be installed already
+#   -s/--skip-existing Skip if the version appears to be installed already
 #
 #   ruby-build options:
 #
@@ -51,6 +52,7 @@ indent() {
 }
 
 unset FORCE
+unset SKIP_EXISTING
 unset KEEP
 unset VERBOSE
 unset HAS_PATCH
@@ -69,6 +71,9 @@ for option in "${OPTIONS[@]}"; do
   "f" | "force" )
     FORCE=true
     ;;
+  "s" | "skip-existing" )
+    SKIP_EXISTING=true
+    ;;
   "k" | "keep" )
     [ -n "${RBENV_BUILD_ROOT}" ] || RBENV_BUILD_ROOT="${RBENV_ROOT}/sources"
     ;;
@@ -128,7 +133,8 @@ PREFIX="${RBENV_ROOT}/versions/${VERSION_NAME}"
 
 # If the installation prefix exists, prompt for confirmation unless
 # the --force option was specified.
-if [ -z "$FORCE" ] && [ -d "${PREFIX}/bin" ]; then
+if [ -d "${PREFIX}/bin" ]; then
+  if [ -z "$FORCE" ] && [ -z "$SKIP_EXISTING" ]; then
     echo "rbenv: $PREFIX already exists" >&2
     read -p "continue with installation? (y/N) "
 
@@ -136,6 +142,12 @@ if [ -z "$FORCE" ] && [ -d "${PREFIX}/bin" ]; then
     y* | Y* ) ;;
     * ) exit 1 ;;
     esac
+  elif [ -n "$SKIP_EXISTING" ]; then
+    # Since we know the ruby version is already installed, and are opting to
+    # not force installation of existing versions, we just `exit 0` here to
+    # leave things happy
+    exit 0
+  fi
 fi
 
 # If RBENV_BUILD_ROOT is set, always pass keep options to ruby-build.