Customize dropdown trigger icon

[GeertHuls]

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[ ] bug report => Search github for a similar issue or PR before submitting
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Current behavior
The p-dropdown component comes with the fa-caret icon as the trigger icon.

Expected behavior
Ability choose another font awesome icon for the dropdown trigger icon.
Example:

What is the motivation / use case for changing the behavior?
To style applications according to brand compliant style guides.

Please tell us about your environment:

• Angular version: 2.0.X
  4.2.4

• PrimeNG version: 2.0.X
  4.2.0-rc.1