Allow alternate config file paths to be specified (CI failure)

[nblumhardt]

It looks like some external package changes have upset CI? https://ci.appveyor.com/project/serilog/serilog-settings-appsettings

[nblumhardt]

This is due to dotnet-test-xunit.exe now loading its own config file earlier in the process, thus thwarting our efforts to override the config file location in the test project.

We might as well support specifying a custom file path to solve this issue:

    .ReadFrom.AppSettings(configPath: "some.config")