Added new option: attributes which are embedded in html tags

[lochbrunner]

In order to use this plugin together with other gulp-plugins such as for example gulp-inline-source, added an option called attributes which adds the specified attributes to the corresponding html tags.

The attributes can be

• a string (the string is added as a attribute (suitable for gulp-inline-source: attributes: 'inline'))
• an object ({a: 'A', b: 'B'} leads to a="A" b="B")
• an array of
  • strings: ( ['a', 'b'] -> a b)
  • objects ( [{name: 'a', value:'A'}, {name: 'b', value:'B'}] -> a="A" b="B" )
  • arrays ( [['a', 'A'],['b', 'B']] -> a="A" b="B")

Tests for this feature are added too.

Please let me know if I should make some additional changes. For example specifying attributes to special files.