JSON compare_config format

[omron93]

Based on napalm documentation the compare_config() should be returning a string - https://napalm.readthedocs.io/en/latest/base.html#napalm.base.base.NetworkDriver.compare_config

Current compare_config implementation for JSON returns list of dictionaries. Also that's missing any formatting so it's not easy for human to review.
Also the current format/data type is not consistent with the return values from compare_config() for on-box set-style diff - which is nicely formatted human readable string.

The simplest fix might be to return pprint.pprint(<diff>) from the JSON diff. With exception of "no diff' for which I think napalm should be returning empty string.
Best fix I guess might be some form of unified-diff style output - lines with '+' prefix for added parts, '-' line prefix for removed parts.