commands: Make commands.Execute return a Response object

commands: Make commands.Execute return a Response object

We have no global `Hugo` object no more (yay!), and there are some external tools that depends on that value.

These tools need to use get that value from `Response.Result`.

Note that `commands.Execute` now also takes the arguments as a string slice. This should also make it easier to use, not having to modify `os.Args`.

This commit also wraps up this particular issue. Phew!

Test coverage in /commands before: 14.4%
Now:  53.5%

Still work to do, now it is at least possible.

Closes #4598