Add auxiliary functions and fix bugs

joyn 0.1.3

• Convert internal data to external data.

joyn 0.1.2

• Add function possible_ids() to identify what variables are suitable for
  uniquely identify the database.

joyn 0.1.1

• Add function is_id() to check whether the table is uniquely identified by
  key variables

• Add function freq_table() as a substitute for janitor::tabyl. This makes it
  more convenient for users who do not have janitor installed.

joyn 0.1.0

Fix bug on by argument when using "=" or "==".

joyn 0.0.1

First Public release