cherry picked themable code to color variables list

[StAmourD]

Split from PR #220 by @cgalvarez

Context var list

• Now bool values are printed correctly as true/false (previously was being printed as 1/0).
• Numeric keys are not enclosed between quotes anymore. This improves readability, since it's far easier to find any variable/key/property.
• Themeable, which improves readability too, since it's far easier to distinguish the type of a value based on its color:
  • The default is to inherit the styles from the current atom theme for the language-php package. This way, one uses the same color codes in the editor and the panels from this package.
  • Now every label span is provided with new classes if it's a property of an object or a key of an array (otherwise it's a variable): syntax--property and syntax--numeric/syntax--string depending on the type.
  • Now every value span is provided with a class with the value type: syntax--array, syntax--bool, syntax--error, syntax--null, syntax--numeric, object, syntax--resource, syntax--string, or syntax--uninitialized.
  • Every span item uses the same syntax-php from the package language-php.
  • The top taxonomies (Locals/Superglobals/User defined constants) have the class syntax--type.

[cchamplin]

LGTM