Fix bug: themes break global $PROMPT_COMMAND variable

Safely append functions to $PROMPT_COMMAND instead of setting it.

themes/sexy/sexy.theme.bash

@@ -43,4 +43,4 @@ function prompt_command() {
   PS1="\[${BOLD}${MAGENTA}\]\u \[$WHITE\]at \[$ORANGE\]\h \[$WHITE\]in \[$GREEN\]\w\[$WHITE\]\$([[ -n \$(git branch 2> /dev/null) ]] && echo \" on \")\[$PURPLE\]\$(parse_git_branch)\[$WHITE\]\n\$ \[$RESET\]"
 }
 
-PROMPT_COMMAND=prompt_command
+safe_append_prompt_command prompt_command