Add ability to select columns from unstructured data

[greg-1-anderson]

Overview

This pull request:

• [ ] Fixes a bug
• [x] Adds a feature
• [ ] Breaks backwards compatibility
• [x] Has tests that cover changes

Summary

Using --fields and --field options, this PR allows individual data items to be selected and extracted from unstructured data.

Examples

$ ./drush sa @wk
'@wk.dev':
  uri: 'http://dev.westkingdom.org'
  root: /Users/ga/local/sca/website
'@wk.live':
  host: example.com
  uri: 'http://westkingdom.org'
  user: www-admin
  root: /var/www/westkingdom/drupal
  paths:
    alias-paths:
      - sites/drush
$ ./drush sa @wk --fields=host,uri
'@wk.dev':
  uri: 'http://dev.westkingdom.org'
'@wk.live':
  host: example.com
  uri: 'http://westkingdom.org'
$ ./drush sa @wk --fields='host as h,uri as u'
'@wk.dev':
  u: 'http://dev.westkingdom.org'
'@wk.live':
  h: example.com
  u: 'http://westkingdom.org'
$ ./drush sa @wk --fields='paths.alias-paths'
'@wk.dev': {  }
'@wk.live':
  alias-paths:
    - sites/drush
$ ./drush sa @wk --fields='paths.alias-paths as .' 
'@wk.dev': {  }
'@wk.live':
  - sites/drush
$ ./drush sa @wk --fields='root' 
'@wk.dev':
  root: /Users/ga/local/sca/website
'@wk.live':
  root: /var/www/westkingdom/drupal
$ ./drush sa @wk --fields='root as .' 
'@wk.dev': /Users/ga/local/sca/website
'@wk.live': /var/www/westkingdom/drupal