Modify clink.arg.new_parser() behaviour

clink.arg.new_parser() now accept variable number of arguments, threated as parser arguments and/or flags, so the user can write in his completion file something like this:
```
local parser = clink.arg.new_parser
some_parser = parser({arg1-1, arg1-2, ...}, {arg2-1, arg2-2, ...}, flag1, flag2...)
```