Consider completing arguments to sub commands when using dotnet 1.x CLI

[lzybkr]

It would be nice for completion to be smarter about the context.

For example, if I type:

dotnet new <TAB>

As currently implemented, when I hit TAB< I get commands like new again even though it doesn't make sense.

Completion should only suggest valid arguments to the new sub-command or valid templates for a new project.

To see one way to do this, see here - this code supports completing service names in the following context:

net start <TAB>

[bergmeister]

@lzybkr I have modified the title now to reflect that the new version posh-dotnet uses now the dotnet CLI API for version 2 of the dotnet CLI. Therefore this issue applies now only to version 1.x of the dotnet cli.