make --quiet option work for all files

[scott-silver]

Currently, the --quiet option only applies to the first file evaluated by solhint.

Say you have a directory with three contracts: contracts/Greeter.sol, contracts/Greeter2.sol and contracts/Greeter3.sol. All three contracts have one error and one warning.

If you run solhint contract/**/*.sol, you'll see all errors and warnings:

But if you run solhint --quiet contract/**/*.sol, you'll see all errors for the first file (Greeter.sol) and errors+warnings for the other two files:

Presumably, the preferred behavior is to hide warnings for all files:

This patch hides warnings for all files when the --quiet flag is passed.