Write drush command to generate and maintain docs

Problem

Certain Webform documentation pages are easier to maintain as MarkDown within the webform module and other documentation page needs to be generated from data and information stored in the Webform module.

Solution

Write drush command to generate and maintain docs

Tasks

• 'drush webform-docs'
• Generate MD
• Add note that document is automatically generated from
• http://cgit.drupalcode.org/webform/tree/docs/DEVELOPMENT-CHEATSHEET.md?h...
• Generate HTML into 'html' sub directory.

Docs

• features WEBFORM-FEATURES.md
• add-ons WEBFORM-ADD-ONS.md
• elements WEBFORM-ELEMENTS.md
• videos WEBFORM-VIDEOS.md
• libraries WEBFORM-LIBARIES.md