auto-detect implementations on missing config file

[michaelficarra]

We should be able to detect the locations of implementations through something similar to which js, which d8, etc. We should also probably run the --version to make sure it is compatible before adding it. This is slightly dangerous because we could be running things that are not ECMAScript implementations.

[bterlson]

We can autodetect names and then run the eshost tests in them to determine which kind of host its in. This would also be handy for not having to remember the host type whenever you add a new host - it could just be autodetected by finding which host type where the tests pass.