Added header objects and defaults

[d1b1]

This feature allow the stream transform to support header objects, that alter the header field and enable default values for missing fields.

var headers = [
{ title: 'My Foo', field: 'foo' },
{ title: 'My Bar', field: 'bar' },
{ title: 'My Bad', field: 'bad', default: '--' }
]

[bilalaamir]

Hey @maxogden is there any progress on this? I actually need to do something similar. I really like this package and don't want to replace it just because of this.