fix #81 refactory struct field parser to make fieldas correct in all cases

Author: huandu

README.md

@@ -134,8 +134,8 @@ We can define a struct type and use field tags to let `Struct` know how to creat
 type ATable struct {
     Field1     string                                    // If a field doesn't has a tag, use "Field1" as column name in SQL.
     Field2     int    `db:"field2"`                      // Use "db" in field tag to set column name used in SQL.
-    Field3     int64  `db:"field3" fieldtag:"foo,bar"`   // Set fieldtag to a field. We can use methods like `Struct#SelectForTag` to use it.
-    Field4     int64  `db:"field4" fieldtag:"foo"`       // If we use `s.SelectForTag(table, "foo")`, columnes of SELECT are field3 and field4.
+    Field3     int64  `db:"field3" fieldtag:"foo,bar"`   // Set fieldtag to a field. We can call `WithTag` to change default tag or call methods like `Struct#SelectForTag` to set tag explicitly.
+    Field4     int64  `db:"field4" fieldtag:"foo"`       // If we use `s.WithTag("foo").Select(table)`, columnes of SELECT are field3 and field4.
     Field5     string `db:"field5" fieldas:"f5_alias"`   // Use "fieldas" in field tag to set a column alias (AS) used in SELECT.
     Ignored    int32  `db:"-"`                           // If we set field name as "-", Struct will ignore it.
     unexported int                                       // Unexported field is not visible to Struct.