Add additional 'using_persistence' global vars

In further testing, it became clear that adding the global variables to the setup and start functions would be useful.

It also seemed to make sense to return boolean values to detect if Persistence has stopped (false) or is in active use (true).

lua/persistence/init.lua

@@ -43,14 +43,15 @@ function M.start()
       autocmd VimLeavePre * lua require("persistence").save()
     augroup end
   ]])
+  vim.g.using_persistence = true
 end
 
 function M.stop()
-  vim.g.using_persistence = nil
   vim.cmd([[
     autocmd! Persistence
     augroup! Persistence
   ]])
+  vim.g.using_persistence = false
 end
 
 function M.save()
@@ -58,15 +59,15 @@ function M.save()
   vim.o.sessionoptions = table.concat(Config.options.options, ",")
   vim.cmd("mks! " .. e(M.get_current()))
   vim.o.sessionoptions = tmp
-  vim.g.using_persistence = e(M.get_current())
+  vim.g.using_persistence = true
 end
 
 function M.load(opt)
   opt = opt or {}
   local sfile = opt.last and M.get_last() or M.get_current()
   if sfile and vim.fn.filereadable(sfile) ~= 0 then
-    vim.g.using_persistence = e(sfile)
     vim.cmd("source " .. e(sfile))
+    vim.g.using_persistence = true
   end
 end