grep.kak: make it possible to search for patterns starting with '-'

[occivink]

Previously if you were to call grep while just selecting something like -f, it would interpret this as a flag and fail.

[losnappas]

Won't this make it impossible to pass flags to the grep program? I like to use e.g. rg -u often.

[occivink]

It shouldn't, the only affected case is when you do :grep with no arguments