Coverlet: Add nightly/dev pre-release package to nuget

[coderpatros]

Not an issue, really just a nice to have. It seems this project has become quite popular already. And it would be good to be able to take advantage of new features without waiting for the next release or making a custom build.

[tonerdo]

This is definitely something I've been meaning to setup. Will try to get to it sometime this week

[StephenMP]

This is pretty easy to do with Travis-CI. You can generate a pre-release version number using the current DateTime and the Travis CI build number (e.g. [Release Version]-ci-[Year][Month][Day]-[TRAVIS_BUILD_NUMBER]). For example, let's say the current release is v1.2.0, the current date is today (May 14, 2018) and currently Travis is on build 5. That would generate the verison number of 1.2.0-ci-20180514-5.

Should be straightforward to add the necessary steps into the build.proj under the Release configuration. You'd just have to make sure to add the NuGet Repository API Key to the Travis Repository Settings (https://docs.travis-ci.com/user/environment-variables/#Defining-Variables-in-Repository-Settings) so you can push it to NuGet.

[MarcoRossignoli]

done https://github.com/tonerdo/coverlet/blob/master/Documentation/ConsumeNightlyBuild.md