Allow multiple asserts on standalone exit_code assertion

[Chemaclass]

Related: #336 (comment)

Enable running an additional assertion over the generated output from assert_exit_code (docs).

Example 1

bashunit -a exit_code "1" "the_command" -a contains "some error"

Would be identical as:

OUTPUT=$(bashunit -a exit_code "1" "the_command")
bashunit -a contains "some error" $OUTPUT