Allow user to supply only part of the version when installing and inform ~ and ^

[giggio]

So, can now do:

dotnet commands install [email protected]

It would be nice if we could only inform parts of the version number, for example 1 would mean 1.0.0, and 1.1 would mean 1.1.0.

So this:

dotnet commands install dotnet-foo@2

Would install version 2.0.0, even if a greater 2.x version exists.

It would also be nice if we could use ^ to mean greatest version with that major. So:

dotnet commands install dotnet-foo@^2.1.3

Would install the largest version that has 2 as the major. So it could match, for example, 2.0.0, 2.1.3, or 2.5.1, whichever exists and is greater.

And ~ to mean greatest version with that major and that minor. So:

dotnet commands install dotnet-foo@~2.1.3

Would install the largest version that has 2 as the major, and 1 as the minor. So it could match, for example, 2.1.0, 2.1.3, or 2.1.9, whichever exists and is greater.