refactor(config.js): Extract config file

* Separate different config source to each files
* Freeze config object

lib/config/default.js

@@ -0,0 +1,92 @@
+'use strict'
+
+module.exports = {
+  domain: '',
+  urlpath: '',
+  port: 3000,
+  urladdport: false,
+  alloworigin: ['localhost'],
+  usessl: false,
+  protocolusessl: false,
+  usecdn: true,
+  allowanonymous: true,
+  allowfreeurl: false,
+  defaultpermission: 'editable',
+  dburl: '',
+  db: {},
+  // ssl path
+  sslkeypath: '',
+  sslcertpath: '',
+  sslcapath: '',
+  dhparampath: '',
+  // other path
+  tmppath: './tmp',
+  defaultnotepath: './public/default.md',
+  docspath: './public/docs',
+  indexpath: './public/views/index.ejs',
+  hackmdpath: './public/views/hackmd.ejs',
+  errorpath: './public/views/error.ejs',
+  prettypath: './public/views/pretty.ejs',
+  slidepath: './public/views/slide.ejs',
+  // session
+  sessionname: 'connect.sid',
+  sessionsecret: 'secret',
+  sessionlife: 14 * 24 * 60 * 60 * 1000, // 14 days
+  staticcachetime: 1 * 24 * 60 * 60 * 1000, // 1 day
+  // socket.io
+  heartbeatinterval: 5000,
+  heartbeattimeout: 10000,
+  // document
+  documentmaxlength: 100000,
+  // image upload setting, available options are imgur/s3/filesystem
+  imageUploadType: 'filesystem',
+  imgur: {
+    clientID: undefined
+  },
+  s3: {
+    accessKeyId: undefined,
+    secretAccessKey: undefined,
+    region: undefined
+  },
+  s3bucket: undefined,
+  // authentication
+  facebook: {
+    clientID: undefined,
+    clientSecret: undefined
+  },
+  twitter: {
+    consumerKey: undefined,
+    consumerSecret: undefined
+  },
+  github: {
+    clientID: undefined,
+    clientSecret: undefined
+  },
+  gitlab: {
+    baseURL: undefined,
+    clientID: undefined,
+    clientSecret: undefined,
+    scope: undefined
+  },
+  dropbox: {
+    clientID: undefined,
+    clientSecret: undefined
+  },
+  google: {
+    clientID: undefined,
+    clientSecret: undefined
+  },
+  ldap: {
+    providerName: undefined,
+    url: undefined,
+    bindDn: undefined,
+    bindCredentials: undefined,
+    tokenSecret: undefined,
+    searchBase: undefined,
+    searchFilter: undefined,
+    searchAttributes: undefined,
+    tlsca: undefined
+  },
+  email: true,
+  allowemailregister: true
+}