Resolves 1641 - Checks whether the prompt is set to be managed or not. (#1723)

This is a new variable that will need to be set on all new prompts and
is not backwards compatible with custom prompts that are not prezto
managed, but use prezto's editor-info functionality. Updated the
README.md with additional information for themes.

modules/prompt/functions/prompt_sorin_setup

@@ -118,6 +118,9 @@ function prompt_sorin_setup {
   # Add hook for calling git-info before each command.
   add-zsh-hook precmd prompt_sorin_precmd
 
+  # Tell prezto we can manage this prompt
+  zstyle ':prezto:module:prompt' managed 'yes'
+
   # Set editor-info parameters.
   zstyle ':prezto:module:editor:info:completing' format '%B%F{7}...%f%b'
   zstyle ':prezto:module:editor:info:keymap:primary' format ' %B%F{1}❯%F{3}❯%F{2}❯%f%b'
@@ -176,3 +179,4 @@ function prompt_sorin_preview {
 }
 
 prompt_sorin_setup "$@"
+# vim: ft=zsh