Add possibility to override Field.jsx

[gerwinbrunner]

I added a way to pass a fieldComponent prop to the Field so that the underlying field can be extended and the render overwritten.

[coveralls]

Pull Request Test Coverage Report for Build 368

• 27 of 27 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 365:	0.0%
Covered Lines:	723
Relevant Lines:	723

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💛 - Coveralls