Add an option to insert attributes

[elchininet]

This pull request implements a new order item property that will add custom attributes to a sidebar item.

Order Items Properties

Property	Type	Required	Description
attributes	String or Object	no	If it is an object it defines a group of attributes that will be applied to the sidebar item. If it is a string, then it makes reference to an object defined in js_variables

Attributes as an object

order:
  - item: 'settings'
    attributes:
      data-my-prop: 'some-prop'
      data-test: 'test-attribute'
      

Attributes as a string (they make reference to a variable in js_variables)

js_variables:
  my_attributes:
    data-my-prop: 'some-prop'
    data-test: 'test-attribute'
order:
  - item: 'settings'
    attributes: 'my_attributes'
      

[coveralls]

coverage: 100.0%. remained the same
when pulling 2120e59 on add_an_option_to_insert_attributes
into f399f39 on master.