Deprecate main array

I'm swayed to deprecate the main array if we come up with a replacement for it.

Though, I really want to state the original requirement of one file extension in the array. And that build tools should still support it since existing packages are using this format. V2 of the spec could remove the mention of this.

/cc @benschwarz @eddiemonge @wibblymat

References

* https://github.com/bower/bower/issues/935
* https://github.com/bower/bower/pull/963