allow for argument prefixing

this plugin is excellent - I'm definitely going to use it with a lot of my common commands. one use I'm already getting great mileage out of is supplanting my [vim-fuzzee](http://github.com/mattsacks/vim-fuzzee) plugin without having to manually tab to the result of the globbed files.

something I thought could be pretty cool is to create mappings that prefix a command to reduce typing common arguments. I've modified the source so that it will allow for this, but I haven't done too much extensive testing into what I've possibly broken.

as an example: `nnoremap <Leader>js :call SkyBison("F app/jav*", 0)`

This will call the F command with `app/jav*<user input>` and allow for completion from the results of the command without having to manually hit <TAB> to see them.