convert argument fmt into dict if argument is a string

FluentRecordFormatter is expecting fmt to be either a callable or a dict. But there is no strict type checking is done to check whether the variable is dict or not.

There are many situations in which `fmt` can have a string value. 
For eg: if logger is loading configuration from a config file using [ConfigParser](https://docs.python.org/2/library/configparser.html) , then there is no way to specify a dict inside the config file.

This change is backward compatible from Python2.7 to Python3.8 since `ast.literal_eval` will work in same way since python 2.6+

fluent/handler.py

@@ -79,6 +79,9 @@ def __init__(self, fmt=None, datefmt=None, style='%', fill_missing_fmt_key=False
                     self._formatter = fmt
                     self.usesTime = fmt.usesTime
                 else:
+                    if type(fmt) == str:
+                        import ast
+                        fmt = ast.literal_eval(fmt)
                     self._fmt_dict = fmt
                     self._formatter = self._format_by_dict
                     self.usesTime = self._format_by_dict_uses_time