Don't clobber scroll offsets

Closes #26.

plugin/sensible.vim

@@ -40,8 +40,12 @@ set ruler
 set showcmd
 set wildmenu
 
+if !&scrolloff
   set scrolloff=1
+endif
+if !&sidescrolloff
   set sidescrolloff=5
+endif
 set display+=lastline
 
 if &listchars ==# 'eol:$'