Add 'flatMap' method #18
Description
The same as the java8 flapMap stream API method.
https://ruby-doc.org/core-2.6/Enumerable.html#method-i-flat_map
{{ message }}
The same as the java8 flapMap stream API method.
https://ruby-doc.org/core-2.6/Enumerable.html#method-i-flat_map