Default missing files to use normal gf (and hence don't force the 'gf' mapping on users.

I was enjoying the basic functionality of this but found it frustrating that my own path and suffixadds values were ignored. That means files within my project's 'src' directory may not be found etc. if I'm using this plugin. This set of changes fixes that so that 'normal gf' is the fallback. To make that work cleanly I changed from mapping keys to exporting commands that the user can map as they see fit (I ended up with <leader>o myself).