Allow maelstrom to be executed from anywhere

[philippgille]

When working on code for one of the scenarios, for example in ~/challenge/echo, you first have to switch to /path/to/maelstrom to then execute ./maelstrom ... --bin ~/challenge/echo/echo ....

This is a bit cumbersome, and can also lead to mixing up the proper binary path (as I did myself here).

With the proposed change, you can stay in ~/challenge/echo and execute /path/to/maelstrom/maelstrom ... --bin ./echo ....

[philippgille]

PS: Tested on Fedora Linux

[aphyr]

Thanks!