Add username/password login and keychain manager

Fix #224

Model/KeychainModel.swift

@@ -0,0 +1,26 @@
+import Foundation
+import KeychainAccess
+
+struct KeychainModel {
+    static var shared = KeychainModel()
+
+    var keychain = Keychain(service: "stream.yattee.app")
+
+    func updateAccountKey(_ account: Account, _ key: String, _ value: String) {
+        keychain[accountKey(account, key)] = value
+    }
+
+    func getAccountKey(_ account: Account, _ key: String) -> String? {
+        keychain[accountKey(account, key)]
+    }
+
+    func accountKey(_ account: Account, _ key: String) -> String {
+        "\(account.id)-\(key)"
+    }
+
+    func removeAccountKeys(_ account: Account) {
+        try? keychain.remove(accountKey(account, "token"))
+        try? keychain.remove(accountKey(account, "username"))
+        try? keychain.remove(accountKey(account, "password"))
+    }
+}