CLI tool: add flag to print ONLY warnings

Description

The current output of the CLI tool includes the lint progress and result for every file. It would be nice to have a way to filter out successful results and only print the warnings.

Repro steps

1. dotnet fsharplint -f "foo.fsproj"

Expected behavior

========== Linting File2.fs ==========
blah
========== Finished: 1 warnings ==========
========== Summary: 1 warnings ==========

Actual behavior

========== Linting File1.fs ==========
========== Finished: 0 warnings ==========
========== Linting File2.fs ==========
blah
========== Finished: 1 warnings ==========
========== Linting File3.fs ==========
========== Finished: 0 warnings ==========
========== Summary: 1 warnings ==========

Known workarounds

Piping the output into another tool, i.e. grep:

dotnet fsharplint -f "foo.fsproj" | grep -Pzo '(?s)=+ Linting (\N*) =+\n(?!=+ Finished: 0 warnings =+\n).*?=+ Finished: [^0] warnings =+\n'

Related information

• Operating system: Windows 10 Pro
• dotnet-fsharplint version: 0.11.1
• .NET Core version: 2.1.505

Start with the dotnet fsharplint -f "foo.fsproj" CLI entry point and trace how lint progress and per-file results are printed. Define the new warnings-only flag and verify it suppresses files with zero warnings while retaining warning output and the summary shown in the expected behavior.