Added NoTypeInference flag to Json and Xml TP

Issue: https://github.com/fsharp/FSharp.Data/issues/656

Note: At the current state of the PR I am looking for 
input if the approach is reasonable and how much
test coverage we need to.

Previously the TypeProvider for Json and XML
tried to infer the real datatype by looking
at input string data. Ie. a string value of
"123" would be inferred as an int.

The inference is sometimes not desirable as seen
here:
https://github.com/fsharp/FSharp.Data/issues/656

NoTypeInference is by default set to false to
keep the previous behavior intact. When setting
NoTypeInference to true the TypeProvider no longer
tries to infer the datatype from input string data.

Unless something is wrong with the tests, they should fail, as there is no expected file for the entry you added. To regenerate you can either run the FSharp.Data.DesignTime as a console app (by renaming Test.fsx to Test.fs), or you can evaluate `generateAllExpected()` in the REPL (https://github.com/fsharp/FSharp.Data/blob/master/tests/FSharp.Data.DesignTime.Tests/SignatureTests.fs#L35)