"Description" help text + general cleanup

Squashed commits:
[f6d094a] PR review changes
[cdb4485] description help text for arguments

- removed unused `help.bannerFor()`
- updated some JSDoc
- option defaults no longer display if default value is an empty string
- rewrote unit tests for `help.help()` function
    - the old tests made it difficult to debug since Mocha would only report truthiness
    - using the updated method, we now compare strings and can easily see any differences as reported by the assertions
    - I also elected to ignore extra whitespace here, because I want to test the content; not the `text-table` module's output.
[8ad7e91] general cleanup: removing some unused vars, updating some JSDoc tags & comments, simplified expression
Merge pull request #534 from boneskull/master

Small updates to "help"
Fix a RunContext so it run only once

There was a race condition causing a generator to run multiple times if
the temp directory rimraf action was too fast.
Feature: RunContext mock prompt by default

Also ensure helper.mockPrompt keep the prompt asynchronous and keep the
origin reference to the initial prompt method when called multiple times.

Fix #539
Merge https://github.com/yeoman/generator
Collect prompt validation errors on generator, #542

This allows to assert validation results for mocked prompt answers.