Add support for 'vaulted' shells in agnoster theme

If a user is currently in a [vaulted](github.com/miquella/vaulted) shell, they
will now have an indication of this in the agnoster prompt, not unlike the
virtualenv prompt. The foreground color will switch if the timer expires (which
usually signifies that the temporary Amazon keys generated by vaulted will be
invalid).

themes/agnoster.zsh-theme

@@ -198,6 +198,23 @@ prompt_virtualenv() {
   fi
 }
 
+# vaulted: current vaulted shell
+prompt_vaulted() {
+  if [[ -z $VAULTED_ENV ]]; then
+    return
+  fi
+  local exp=$(echo $VAULTED_ENV_EXPIRATION | sed 's/Z/+0000/')
+  local valid_until=$(date -j -f %Y-%m-%dT%H:%M:%S%z $exp +%s)
+  local bg=009 #orange
+  local fg=black
+  if [[ $valid_until -lt $(date +%s) ]]; then
+    fg=blue
+  fi
+  if [[ -n $VAULTED_ENV ]]; then
+    prompt_segment $bg $fg "$VAULTED_ENV"
+  fi
+}
+
 # Status:
 # - was there an error
 # - am I root
@@ -217,6 +234,7 @@ build_prompt() {
   RETVAL=$?
   prompt_status
   prompt_virtualenv
+  prompt_vaulted
   prompt_context
   prompt_dir
   prompt_git