fix: don't freak out when not passed a finders config

As reported here:

    wincent/command-t#393 (comment)

if `options.finders` is not set to (at least an empty) table, we freak
out with:

    lua/wincent/commandt/init.lua:343: bad argument #1 to 'pairs' (table expected, got nil)

lua/wincent/commandt/init.lua

@@ -340,7 +340,7 @@ commandt.setup = function(options)
   require_string('selection_highlight')
   require_boolean('smart_case')
 
-  for name, finder in pairs(options.finders) do
+  for name, finder in pairs(options.finders or {}) do
     require_function_or_string('finders.' .. name .. '.command', nil, {
       finders = {
         [name] = {