Support extra mappings when modal.

When g:filepirate_is_modal is enabled, allow custom mappings to be specified with the variables g:filepirate_map_extra_normal and g:filepirate_map_extra_insert. This effectively defines private 'modes' for File Pirate in which you can completely alter the behaviour of vim.

Practically speaking it means you can set up keyboard shortcuts to jump to specific files when in normal mode. There may be other cool uses, too.