Update index.d.ts (#1729)

Hoist type defs from the logform module.

Currently there isn't a way to write correct type definitions for
when e.g creating a format since the types for such objects are
declared in `logform`.
This patch move logform type defs so that they can be used by
user code - For example by doing:

`import { Logform } from 'winston'; const f: Logform.Format;`

index.d.ts

@@ -14,6 +14,7 @@ import * as Transports from './lib/winston/transports/index';
 declare namespace winston {
   // Hoisted namespaces from other modules
   export import format = logform.format;
+  export import Logform = logform;
   export import config = Config;
   export import transports = Transports;