Adding 'dir' command to display project root

Using Exercism on different machines I ofter forget in what directory I saved the exercises, so this would be somewhat helpfull to me. Don't know if many others have this 'problem'.

You can use it like this

    cd `exercism dir`

or make an alias

    alias e="cd `exercism dir`"

Making a `cd` command would have been a better sollution I think, but that is near impossible to make it work crossplatform (I read some Windows related stuff in the source) and even on UNIX I imagine it's not trivial.