Adding options for path offsets

I added two variables for path offset. See

    doc/vim-annotations.txt

for details. Basically I have rigged things so all temp files when checking
  
    path/to/target

are generated in a directory 

   path/to/.liquid/

instead of cluttering the top-level path/to directory. Now you can have vim-annotations either
behave as before OR if you prefer, in the "offset" directory.