Rely on Inquirer for mocking prompt rather than duplicating logic

As a side effect of relying on Inquirer, each Terminal Adapter now have
a self contained Inquirer context to prevent cross pollution between
multiple generators being runned at the same time.

This change also remove how we previously attached prompts validation
errors to the question object.
Merge pull request #659 from SBoudrias/mock-inquirer

Rely on Inquirer for mocking prompt rather than duplicating logic
cleanup: Remove usage of process.nextTick