Enable read config with defaults

This pull request enables to read configs with defaults like this:

```
db.default.driver="org.h2.Driver"
db.default.user="app_user"

dev {
  db.default.url="jdbc:h2:dev"
  db.default.password="pwd"
}
test {
  db.default.url="jdbc:h2:test"
  db.default.password="pwd"
}
prod {
  db.default.driver="org.postgresql.Driver"
  db.default.url="jdbc:postgresql://localhost:5432/scalikejdbc"
  db.default.password=${?DATABASE_PASSWORD}
}
```