Adding the collapse delimiter as parameter

[brunj7]

Thanks for this great package! This is a suggestion:

It will be great if the user was able to choose the delimiter as a parameter for 'flatten()and thuswrite_as_csv()`

Let me know if it sounds useful. Happy to do a PR with this small change implemented

[mkearney]

Would happily accept this as a PR, but I would want to check how it gets implemented in save_as_csv() via flattening/deflattening–i.e., read_as_csv() assumes $mentions_user_id is space delim so ideally it'd be able to detect (obvi ones: space, comma, plus-sign) and allow user spec via delim argument.