Allow custom configs to be passed to `spawn`.

The main point is to allow passing other `std` Streams to output command
content
Fix base generator tests

Conflicts:
	test/base.js
Simplify env#namespace and add missing tests
Rely on NPM ls for Env#lookup functionnality

Also remove a lot of noisy "too much configs" code that growed the
complexity of the code paths.

Conflicts:
	test/base.js
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