Add --no-push-path option (#2526)

In some advanced shell setups, the order of custom-added PATH entries is important.
We disregard it by default, always pushing shims to the front of PATH,
to ensure that Pyenv works even in poorly maintained shell environments
and with minimal hassle for non-export users
(an attempt to do the opposite (#1898) has ended in a disaster).
Some advanced users are however ready and able to carefully maintain their environment
and deal with breakages and inconvenience. This option is for them.

COMMANDS.md

@@ -386,11 +386,12 @@ List existing pyenv shims.
 
 Configure the shell environment for pyenv
 
-    Usage: eval "$(pyenv init [-|--path] [--no-rehash] [<shell>])"
+    Usage: eval "$(pyenv init [-|--path] [--no-push-path] [--no-rehash] [<shell>])"
 
       -                    Initialize shims directory, print PYENV_SHELL variable, completions path
                            and shell function
       --path               Print shims path
+      --no-push-path       Do not push shim to the start of PATH if they're already there
       --no-rehash          Add no rehash command to output     
 
 ## `pyenv completions`