Allow custom transforms to participate in the main-thread pre-compilation

[jamestalmage]

We currently support pre-compilation of ES2015 using Babel in the main thread. This means the child processes don't pay the penalty of loading up Babel for every thread.

TypeScript and CoffeScript users can use require hooks (i.e. --require coffe-script/register), but then they pay that cost for every forked process (increased require times and memory usage).

We could allow other transforms to participate via some sort of plugin API. This would allow TypeScript and CoffeeScript users to realize the same speed increase and reduced memory footprint.

Custom plugins for other transforms would need to be developed. transform-chain is something I built with this in mind and could be helpful.

The decision we need to make is:

1. Should we do this?
2. Should we do it now or after 1.0.0?
3. What should the API / config look like.

[jamestalmage]

Also, this would allow transforms to participate in transform caching. (transform plugins would need to expose a cache-salt in order to participate).

[sindresorhus]

I think we should defer this to after 1.0.0. We already have a lot on our plates and this is non-essential to our core features and use-cases.

[billyjanitsch]

@jamestalmage In its current form, does --require transform test code, or only imports thereof? (i.e. does --require coffee-script/register allow me to write my tests in Coffeescript?)

[jamestalmage]

For tests you need #631 (comment) to land first

[novemberborn]

Our precompilation plans can be found here: https://github.com/avajs/ava/blob/master/docs/specs/001%20-%20Improving%20language%20support.md