fix: Add id field to match the problem (#24)

Author: jdneo

lib/core.js

@@ -92,7 +92,7 @@ core.getProblem = function(keyword, cb) {
     keyword = Number(keyword) || keyword;
     const metaFid = file.exist(keyword) ? Number(file.meta(keyword).id) : NaN;
     const problem = problems.find(function(x) {
-      return x.fid === keyword || x.name === keyword || x.slug === keyword || x.fid === metaFid;
+      return x.id === keyword || x.fid === keyword || x.name === keyword || x.slug === keyword || x.fid === metaFid;
     });
     if (!problem) return cb('Problem not found!');
     core.next.getProblem(problem, cb);