FEATURE: Add new option for specifying .NET type to express Json numbers: --generate-json-number-as = double | float | decimal with a default of double.

[shaopeng-gh]

FEATURE: Add new option for specifying .NET type to express Json numbers: --generate-json-number-as = double | float | decimal with a default of double. (before change it was fixed as double.)

Background: The difference can be seem here
integer
number

This change added the option for user to generate the C# class double | float | decimal as suitable, for Json number.
Before this change, there is no option and only produce C# class double for Json number.
(Whether the class of the choice is suitable or not, is outside of the scope of this code generation tool,
and is decided by the user of the tool,
according to the target project to put the generated code in.
e.g. the target project use decimals, if our tool only support double, they will need to do the cast back and forth.)

[michaelcfanning]