Add support for showing what virtualenv is currently active

This works with virtualenvwrapper to detect when a virtualenv is
activated and adds "(<name>)" before the path that's being displayed.

themes/doubletime/doubletime.theme.bash

@@ -25,13 +25,20 @@ doubletime_scm_prompt() {
   fi
 }
 
+virtualenv_prompt() {
+  if [ ! -z "$VIRTUAL_ENV" ]
+  then
+    echo "(`basename $VIRTUAL_ENV`) "
+  fi
+}
+
 prompt_setter() {
   # Save history
   history -a
   history -c
   history -r
   PS1="
-\t $(scm_char) [\[$THEME_PROMPT_HOST_COLOR\]\u@${THEME_PROMPT_HOST}\[$reset_color\]] \w
+\t $(scm_char) [\[$THEME_PROMPT_HOST_COLOR\]\u@${THEME_PROMPT_HOST}\[$reset_color\]] $(virtualenv_prompt)\w
 $(doubletime_scm_prompt)\[$reset_color\] $ "
   PS2='> '
   PS4='+ '