fix: --subscribed=false should unsubscribe users

[notnmeyer]

i noticed the --subscribed flag wasn't unsubbing. the flag actually needs to be a string so we can parse the intent. previously, omitting the flag was false and including it was true, which was not exactly what we wanted. this works as expected now.

➜ task -- contacts find -e nate+1@loops.so -ojson | jq -r ".[].subscribed"
true

➜ task -- contacts update -e nate+1@loops.so -s false
Updated.

➜ task -- contacts find -e nate+1@loops.so -ojson | jq -r ".[].subscribed"
false