create spread_all to automatically spread all keys

[jeremystan]

Should work like:

'{"a": 1, "b": "x", "c": true}' %>% spread_all_values

• Should not affect the state of the JSON object
• Should work with nested objects
• Should take a sep argument used to separate key names when objects are nested
• Should just ignore arrays automatically
• NULLs should be cast to NA