Rate limitation function, ct_auth() function

[untergeekDE]

Just a bit of better housekeeping so far:

• ct_auth(token) stores an API token in the local .REnviron file (and returns an error if not valid)
• ct_api_limit() sets a delay matching the maximum number of API calls per minute (6 per default)
• ct_get_links() and ct_get_posts() functions updated to use token if set, wait after API call
• TODOs noted

[untergeekDE]

• Additional housekeeping
• function ct_get_fb_post(url)
• Helper functions for querying instagram posts