guess output format based on output file extension

check for empty output file

default to .txt if no file extension provided

change default output format back to txt

add tests

update man page

fix default output format

directly return output dumper

use keys from dumpers in key error message

fix man pages

prevent output format from being overridden

use first dumper key as default format

update messages and formatting

update help message for format argument

specify list type in InvalidOutputFormat exception

make default output format more apparent

update messages