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.

https://github.com/sstephenson/rbenv/issues/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.
Load local zshenv config