feat: add new upgrade type 'exist'

resolve #27

Author: Val-istar-Guo

src/formatHandlers.js

@@ -24,5 +24,6 @@ module.exports = file => {
   })
 
   if (file.upgrade === 'merge') effectiveHandlers.push(handlers.merge)
+  if (file.upgrade === 'exist') effectiveHandlers.push(handlers.exist)
   return { ...file, handlers: effectiveHandlers }
 }

src/handlers/exist.js

@@ -0,0 +1,11 @@
+const createHandler = require('./createHandler')
+const readTargetFile = require('./readTargetFile')
+
+
+
+module.exports = createHandler(file => {
+  file = readTargetFile(file)
+
+  if (file.targetFile.exist) file.content = file.targetFile.content
+  return file
+})

src/handlers/index.js

@@ -1,6 +1,7 @@
 const merge = require('./merge')
+const exist = require('./exist')
 const mustache = require('./mustache')
 const extractArea = require('./extractArea')
 
 
-module.exports = { merge, mustache, extractArea }
+module.exports = { merge, mustache, extractArea, exist }