feat: full syntax for strftime, close #177

- strftime syntax: <flags><width><modifier><conversion>
- new conversions: %n, %t
- fixed conversions: %N
- flags: _ ^ - 0 # :
- modifiers: E, O (ignored)

test/integration/liquid/fs-option.ts

@@ -12,7 +12,7 @@ describe('LiquidOptions#fs', function () {
     engine = new Liquid({
       root: '/root/',
       fs
-    })
+    } as any)
   })
   it('should be used to read templates', function () {
     return engine.renderFile('files/foo')