Support list / tuple of strings for format

[nhairs]

Many other JSON formatting libraries allow or directly use a list of strings for the format. This suggests that it might be worth us adding in support for it.

Pros:

• No parsing means no testing for a correct format - we can directly use the list.
• Allows for clearer config when using dictConfig with the () user-defined object indicator, or instantiating within Python itself.

Cons:

• Must always use the () custom class indicator when using dictConfig.
• Not compatible with fileConfig.
• Not compatible with other formatters