Add only trusted projects' bin directory to $PATH

Assuming the binstubs for a project are in the local bin/ directory, you
can even go a step further to add the directory to shell $PATH so that
rspec can be invoked without the bin/ prefix:

    export PATH="./bin:$PATH"

However, doing so on a system that other people have write access to
(such as a shared host) is a security risk.

rbenv/rbenv#309

Put this in `zshenv` because:

http://zsh.sourceforge.net/Intro/intro_3.html

> `.zshenv' is sourced on all invocations of the shell, unless the -f
> option is set. It should contain commands to set the command search
> path.

zshenv

@@ -0,0 +1,2 @@
+# mkdir .git/safe in the root of repositories you trust
+export PATH=".git/safe/../../bin:$PATH"