Add option for installing prerelease packages to dotnet add package

[livarcocc]

From @svick on June 30, 2017 10:48

Currently, when I want to add a prerelease package to a project, I have to explicitly specify its version. For example: dotnet add package System.Memory does not work, but dotnet add package System.Memory -v 4.4.0-preview2-25405-01 does.

This is inconvenient when I want to install the latest version of the package, even if it's prerelease. So I propose that a new --prerelease option should be added to dotnet add package, which would allow installing prerelease packages without having to explicitly specify package version.

This is similar the the -IncludePrerelease option to Install-Package.

Copied from original issue: dotnet/cli#7043

[mishra14]

@emgarten restore should consider supporting --pre-release as we have discussed in the past.

[mishra14]

Closing as dupe of #4699