Rethink `extensions` option

Are you ever going to have a chain of transforms on anything but `.js` extensions?
Even if you did, would you want a "chain" per extension?

Once you have converted `.coffee` => `.js`, you probably want to start at the top of the `.js` chain, not just apply the `.js` transforms that are downstream of wherever `coffee-script/register` happens to be in the chain.

Maybe there is no need for the concept of "extensions" in this lib. We already have the `match` option, which could achieve the same thing.

Start by reviewing how the existing `extensions` and `match` options work and how transform chains are selected. The issue does not identify files, tests, or a settled direction; done would require deciding whether to remove `extensions` or redefine its chaining semantics, then documenting and validating that decision.

Written by the indexing model from the issue text.