Added new config option: decode_numbers_as_strings to decode large numbers as strings

setting = cjson.decode_numbers_as_strings([setting])
"setting" must be a boolean. Default: false.
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Lua CJSON will parse numbers as strings and return them as strings if
this setting is true.

A typical use case may be for JSON with 64bit numbers and you don't want
to loose precision for those fields as they're converted to Lua_number
which is typically a double.

Available settings:

true: Decode _numbers_ as a string.
false: Decode _numbers_ as Lua numbers. This is the default setting.