respect _Z_NO_PROMPT_COMMAND environment variable

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

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