Add --prefer-released option to NuGetDiffCommand (#30)

* Add a --prefer-release to remove prerelease

This is useful if you want to include prerelease, but also
want to select release over prerelease as previews change
APIs because they are not official.

* Update PackageVersionsTests.cs

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Co-authored-by: Matthew Leibowitz <mattleibow@live.com>

Update version to stable 1.4.1

Update version to stable 1.4.0

Just do the diff of the latest

Better help

Added some more features (#8)

* generating API info
* diff-ing from an API info
* merging assemblies

Add a command to diff single assemblies (#7)

* Add a command to diff single assemblies
* Make the command write to a file/console
* Let's do v1.2

Build using .net core 3.0 (#5)

* Build using .net core 3.0
* Only build master branch
* Make sure to build .net core 2 and 3 tools
* Fix the tests for new APIs