respect _Z_NO_PROMPT_COMMAND environment variable

For those who wish to set their PROMPT_COMMAND manually. Closes #44.

README

@@ -43,6 +43,7 @@ NOTES
         Set $_Z_CMD in .bashrc/.zshrc to change the command (default z).
         Set $_Z_DATA to change the datafile (default $HOME/.z).
         Set $_Z_NO_RESOLVE_SYMLINKS to prevent symlink resolution.
+        Set $_Z_NO_PROMPT_COMMAND if you're handling PROMPT_COMMAND yourself.
         (These settings should go before the lines below.)
 
        Put something like this in your $HOME/.bashrc:
@@ -122,9 +123,12 @@ ENVIRONMENT
        to any existing PROMPT_COMMAND. zsh users need to manually set precmd()
        as described in the installation instructions.
 
+       The  environment  variable $_Z_NO_PROMPT_COMMAND can be set if you want
+       to handle PROMPT_COMMAND yourself.
+
 
 FILES
-       Data  is  stored  in  $HOME/.z.  This  can be overridden by setting the
+       Data is stored in $HOME/.z. This  can  be  overridden  by  setting  the
        $_Z_DATA environment variable.
 
        A man page (z.1) is provided.

z.1

@@ -45,6 +45,8 @@ Optionally:
  Set \fB$_Z_DATA\fR to change the datafile (default \fB$HOME/.z\fR).
 .br
  Set \fB$_Z_NO_RESOLVE_SYMLINKS\fR to prevent symlink resolution.
+.br
+ Set \fB$_Z_NO_PROMPT_COMMAND\fR if you're handling \fBPROMPT_COMMAND\fR yourself.
 .br
  (These settings should go before the lines below.)
 .P
@@ -126,8 +128,11 @@ its database. Specifically it prepends:
 .P
  \fB_z --add "$(pwd -P 2>/dev/null)" 2>/dev/null;\fR
 .P
-to any existing \fbPROMPT_COMMAND\fR. zsh users need to manually set
+to any existing \fBPROMPT_COMMAND\fR. zsh users need to manually set
 \fBprecmd()\fR as described in the installation instructions.
+.P
+The environment variable \fB$_Z_NO_PROMPT_COMMAND\fR can be set if you want to
+handle \fRPROMPT_COMMAND\fR yourself.
 
 .SH FILES
 Data is stored in \fB$HOME/.z\fR. This can be overridden by setting the

z.sh

@@ -6,6 +6,8 @@
 #   * optionally:
 #     set $_Z_CMD in .bashrc/.zshrc to change the command (default z).
 #     set $_Z_DATA in .bashrc/.zshrc to change the datafile (default ~/.z).
+#     set $_Z_NO_RESOLVE_SYMLINKS to prevent symlink resolution.
+#     set $_Z_NO_PROMPT_COMMAND if you're handling PROMPT_COMMAND yourself.
 #   * put something like this in your .bashrc:
 #     . /path/to/z.sh
 #   * put something like this in your .zshrc:
@@ -184,9 +186,11 @@ alias ${_Z_CMD:-z}='_z 2>&1'
 if complete &> /dev/null; then
  # bash tab completion
  complete -C '_z --complete "$COMP_LINE"' ${_Z_CMD:-z}
- # populate directory list. avoid clobbering other PROMPT_COMMANDs.
- echo $PROMPT_COMMAND | grep -q "_z --add"
- [ $? -gt 0 ] && PROMPT_COMMAND='_z --add "$(pwd '$_Z_RESOLVE_SYMLINKS' 2>/dev/null)" 2>/dev/null;'"$PROMPT_COMMAND"
+ [ "$_Z_NO_PROMPT_COMMAND" ] || {
+  # populate directory list. avoid clobbering other PROMPT_COMMANDs.
+  echo $PROMPT_COMMAND | grep -q "_z --add"
+  [ $? -gt 0 ] && PROMPT_COMMAND='_z --add "$(pwd '$_Z_RESOLVE_SYMLINKS' 2>/dev/null)" 2>/dev/null;'"$PROMPT_COMMAND"
+ }
 elif compctl &> /dev/null; then
  # zsh tab completion
  _z_zsh_tab_completion() {