feat(import): add shouldIgnore config callback

shouldIgnore callback is called before the file is read and gives an
opportunity to stop the import process
Useful in an environment where files cannot be moved around in the
importation directory

src/import/import.js

@@ -17,17 +17,25 @@ exports.import = async function (database, importName, file, options) {
     const parsedFilename = path.parse(file);
     const filedir = parsedFilename.dir;
     const filename = parsedFilename.base;
+    let config = getConfig(database);
+    const couch = Couch.get(database);
+    const shouldIgnore = verifyConfig(config, 'shouldIgnore', null, true);
 
+    // Give an opportunity to ignore before even reading the file
+    if(shouldIgnore) {
+        const ignore = await shouldIgnore(filename, couch, filedir);
+        if(ignore) {
+            debug.debug(`Ignore file ${file}`);
+            return;
+        }
+    }
     let contents = fs.readFileSync(file);
 
-    let config = getConfig(database);
     if (!config.import || !config.import[importName]) {
         throw new Error(`no import config for ${database}/${importName}`);
     }
     config = config.import[importName];
 
-    const couch = Couch.get(database);
-
     const info = {};
 
     let isParse = false;