Ship formatters in separate nuget package

[vhatsura]

What issue does this PR address?
#176

Does this PR introduce a breaking change?
No

Please check if the PR fulfills these requirements

• The commit follows our guidelines
• Unit Tests for the changes have been added (for bug fixes / features)

Other information:

[mivano]

Thanks for the PR. Looks good, but just one question and that might just be my lack of Nuget knowledge; should the Serilog.Sinks.Elasticsearch.nuspec file not need a reference to the newly created nuget formatter package?

Next to that; can you also update the readme.md file with some details?

Thanks!

[vhatsura]

@mivano, you're right regarding nuspec file, however, packages are packed by dotnet pack command, which is clever and understand that Serilog.Formatting.Elasticsearch should be added to dependency graph inside nuspec file. You can check it by yourself from generated nuget package in AppVeyor.

Regarding Readme.md, I'll do it, but could clarify which info you want to see, please?

[mivano]

Thanks! I do indeed see the nuget packages referenced when looking in NuGet Explorer. I merged to dev and will prepare a final package.

[vhatsura]

Got it, thanks. I started to write some details regarding new package. If it possible, it would be good wait changes before final package will be released.

[mivano]

Looks like the master build was skipped due to some condition. I will await your changes and will try again with those details.