Use swapcase for Unicode-aware case swapping

Before:

    $ echo 'aaaᾷᾷᾷ' | node index.js
    aAaΑ͂ΙᾷΑ͂Ι

After:

    $ echo 'aaaᾷᾷᾷ' | node index.js
    aAaᾶιᾷᾶι