further bundle exec completion

examples:
`bundle exec rsp` + TAB => `bundle exec rspec`
`bundle exec rspec --no` + TAB => `bundle exec rspec --no-color`

completion/available/bundler.completion.bash

@@ -21,6 +21,9 @@ __bundle() {
       if [[ -z $bundle_command ]]; then
           options="$options --version --help"
       fi
+  elif [[ $bundle_command = "exec" ]]; then
+      _bundle_exec_completions
+      return
   else
       if [[ -z $bundle_command || $bundle_command = help ]]; then
           options="help install update package exec config check list show
@@ -53,3 +56,10 @@ __bundle_get_command() {
 
 complete -F __bundle -o bashdefault -o default bundle
 # vim: ai ft=sh sw=4 sts=2 et
+
+_bundle_exec_completions()
+{
+    if [[ $COMP_CWORD = 2 ]]; then
+        COMPREPLY=($(compgen -c -- "$2"))
+    fi
+}