Setup code coverage

[lilith]

I've been unable to get OpenCover working (despite enabling full debug builds). If anyone can find a way to get this running it would be super helpful

OpenCover.Console.exe -register:user -target:"C:/Program Files/dotnet/dotnet.exe" -targetargs:"test tests\Imageflow.Test\Imageflow.Test.csproj" -output:".\coverage.xml" -oldstyle

C:\Users\lilith\imazen\imageflow-dotnet>OpenCover.console.exe -version
OpenCover version 4.7.922.0

C:\Users\lilith\imazen\imageflow-dotnet>OpenCover.Console.exe -register:user -target:"C:/Program Files/dotnet/dotnet.exe" -targetargs:"test tests\Imageflow.Test\Imageflow.Test.csproj" -output:".\coverage.xml" -oldstyle
Executing: C:\Program Files\dotnet\dotnet.exe
C:\Program Files\dotnet\sdk\3.0.100\Sdks\NuGet.Build.Tasks.Pack\build\NuGet.Build.Tasks.Pack.targets(198,5): warning NU5125: The 'licenseUrl' element will be deprecated. Consider using the 'license' element instead. [C:\Users\lilith\imazen\imageflow-dotnet\src\Imageflow\Imageflow.Net.csproj]
Test run for C:\Users\lilith\imazen\imageflow-dotnet\tests\Imageflow.Test\bin\Debug\netcoreapp3.0\Imageflow.Test.dll(.NETCoreApp,Version=v3.0)
Microsoft (R) Test Execution Command Line Tool Version 16.3.0
Copyright (c) Microsoft Corporation.  All rights reserved.

Starting test execution, please wait...

A total of 1 test files matched the specified pattern.

Test Run Successful.
Total tests: 10
     Passed: 10
 Total time: 1.1722 Seconds
Committing...
No results, this could be for a number of reasons. The most common reasons are:
    1) missing PDBs for the assemblies that match the filter please review the
    output file and refer to the Usage guide (Usage.rtf) about filters.
    2) the profiler may not be registered correctly, please refer to the Usage
    guide and the -register switch.

[lilith]

We can re-add this to readme when it's set up:
[![Coverage Status](https://codecov.io/gh/imazen/imageflow-dotnet/branch/master/graph/badge.svg)](https://codecov.io/gh/imazen/imageflow-dotnet)