Print warning when deprecated properties are used.

[troydai]

We keep supporting code and resources in project.json for the sake of compatibility. They should be gradually retired so as to simplify the parsing logic.

[davidfowl]

Awesome! We need something for design time host

[troydai]

Developing on this branch. https://github.com/aspnet/XRE/tree/trdai/issue1292

Feature scope:

1. Returns warnings on Build and Bundle command;
2. Shows warnings in VS.

[troydai]

Change of scope: item 2 will be traced on #1330
PR related to this issue will only impact build and bundle command.

[troydai]

Fixed in 41ae546
Moving on to #1330