Add screen prompt

Add a new bullet showing the current screen session.

GNU Screen is a easy to use window manager.
For more information visit: http://www.gnu.org/software/screen/

bullet-train.zsh-theme

@@ -23,6 +23,7 @@ if [ ! -n "${BULLETTRAIN_PROMPT_ORDER+1}" ]; then
     custom
     context
     dir
+    screen
     perl
     ruby
     virtualenv
@@ -275,6 +276,17 @@ else
   ZSH_THEME_GIT_PROMPT_DIVERGED=$BULLETTRAIN_GIT_PROMPT_DIVERGED
 fi
 
+# SCREEN
+if [ ! -n "${BULLETTRAIN_SCREEN_BG+1}" ]; then
+  BULLETTRAIN_SCREEN_BG=white
+fi
+if [ ! -n "${BULLETTRAIN_SCREEN_FG+1}" ]; then
+  BULLETTRAIN_SCREEN_FG=black
+fi
+if [ ! -n "${BULLETTRAIN_SCREEN_PREFIX+1}" ]; then
+  BULLETTRAIN_SCREEN_PREFIX="⚙"
+fi
+
 # COMMAND EXECUTION TIME
 if [ ! -n "${BULLETTRAIN_EXEC_TIME_ELAPSED+1}" ]; then
   BULLETTRAIN_EXEC_TIME_ELAPSED=5
@@ -538,6 +550,14 @@ prompt_aws() {
   fi
 }
 
+# SCREEN Session
+prompt_screen() {
+  local session_name="$STY"
+  if [[ "$session_name" != "" ]]; then
+    prompt_segment $BULLETTRAIN_SCREEN_BG $BULLETTRAIN_SCREEN_FG $BULLETTRAIN_SCREEN_PREFIX" $session_name"
+  fi
+}
+
 prompt_time() {
   if [[ $BULLETTRAIN_TIME_12HR == true ]]; then
     prompt_segment $BULLETTRAIN_TIME_BG $BULLETTRAIN_TIME_FG %D{%r}