Read shell config from file instead of heredoc

As a thank you for maintaining Hitch, I thought I'd contribute a change that came up for us while provisioning a remote pairing server.  There's now a separate bash script that's a little easier to use with Puppet -- it can be installed or verified to be installed as a single file resource.  We were going to separate this just for ourselves, but thought others might find it useful.

Thanks again for making Hitch!

Ben

**Summary**

* New file `hitch.sh` which contains the shell script previously in a heredoc
* New command `hitch --setup-path` for users who want to `cp` or `ln` the file elsewhere.
* Added some notes to the README about how to use this.