feat: add string tag & json.Number

[ziposcar]

feat: add string tag & json.Number

Hello, here are two functional changes. Thanks for reviewing it.

• Add a new cmd option -S -stringtag. Allow users to let our tool to try to add a ,string tag into string type fields that look like numbers.
• Use json.Number interface to determine whether the field is a float or an integer.

And I wrote a test function for example. Do you think there is anything else I should add? I would be gladly to add them. :)