feat: use 'urfave' CLI library for cmd/faucet

[meowsbits]

The cmd/faucet/main.go program currently uses the standard lib flag package, while other programs -- notably cmd/geth -- use gopkg.in/urfave/cli.v1. The faucet program could be improved by using the urfave package. For example, flag exclusivity checks would become easier by being able to import and use cmd/utils#CheckExclusive.