Guide users in providing the right config

[ManasJayanth]

Specifying certain configs can get ambiguous at times.

"modes": "best exe",
"modes": ["best", "exe"],
"modes": ["best exe", "best object"]

The last one isn't supported yet, but seems perfectly valid. While it can always be supported,
JSON can make many such configurations possible and it would be nice to guide the user to the most meaningful syntax.