Add mechanism choco package to describe whether it requires elevation

In .nuspec there is xml tag named `tags`. That can be freely used to describe some custom information, which chocolatey uses.

I would propose to add special parameter `requires_elevation`, which in a turn could specify whether package requires elevation to be installed by default.

See following example:

```



testpackage
1.0.0.0
Sample package
Author

requires_elevation;





```

There is also possibility for choco to change install location using `/params` - but then it would make sense that end-user will also specify whether elevation is needed or not. Choco could take some `--noelevation` to supress automatic process elevation.

Coechoes with https://github.com/chocolatey/choco/issues/2096.