Add variant of jsonTool to generate FromJSON instances (fixes #18)

I've taken the simple option of directly reusing FromJSONWithErrs. This means that FromJSON instances will inherit the more liberal behaviour for basic data types (e.g. accepting quoted integers).

@cdornan shall I put together a release including this, and is there anything else you'd like included? I could at least squash #21 and the easy parts of #6...