Allow to define query as positional argument

Currently in my configuration I setup `let g:grepper = { 'prompt': 0 }` and mapping `nnoremap / :Grepper -query `. It would be really nice if I could drop the `-query` part from mapping (i.e. allow query to be accepted as positional argument to `:Grepper`, e.g. `:Grepper foobar`. This could be available as default or an option. Thank you!

Start by tracing the :Grepper command and how its current -query argument is parsed, using the example mapping and g:grepper configuration in the issue as the expected entry points. Done means :Grepper foobar accepts foobar as the query, while the existing -query form continues to work or the chosen default or option is documented.

Written by the indexing model from the issue text.