It should be configurable whether string fields containing digits should change to numbers

[simonlkh]

I got some data in MongoDB storing as string with digits only as code, e.g. "01" as a code. When using the plugin and transfer data to ES, it changed to integer 1, which is completely different to my data.

It should be configurable which fields would apply type converting action because it is not the same meaning with 1 and "01" in many cases.