Allow to specify runtime dependencies in project.json

[forki]

Is it possible to specify runtime dependencies in project.json?

#2071 shows that it would be useful to give users a way to override resolver decisions when needed.
Also in some cases we need to add additional runtime dependencies that are not expressed in compile time dependencies.

[yishaigalatzer]

@ericstj can you help track the bug that is the duplicate of this issue?

[ericstj]

That'd be this one:
#1660
Spec is here:
https://github.com/dotnet/cli/issues/465