update project's url
rename group-of-two->word to group-of-two->words in test
rename group-of-two->word to group-of-two->words
rename digit->word to digit->words
rename group-of-three->word to group-of-three->words in tests
rename group-of-three->word to group-of-three->words
rename number->word-helper to number->words*

add separator in link-groups function
add separator to link-groups

add optional arg separator to number->words*
add separator to number->words*
update README