Add output flag

[gandarez]

This PR adds new output flag to change output from raw string to json or vice-versa. The default value is text. Only --today and --today-goal can benefit from this flag yet. In addition today-hide-categories has been changed from bool to string to fully support taking precedence over any other setting as it's a known bug.

Testing

# print today as json
./build/wakatime-cli-darwin-amd64 --output json --today

# print today as raw-json (original payload)
./build/wakatime-cli-darwin-amd64 --output raw-json --today

# print today as text
./build/wakatime-cli-darwin-amd64 --output text --today
# or without flag
./build/wakatime-cli-darwin-amd64 --today