[PROPOSE] add file writable check in #configure

`--config-test` is very useful, but it does not check file permission. This proposition is to make a rule to check file permissions in #configure method of each plugin because I thought it is impossible to check file permissions for each configuration parameter of each plugin from Fluentd side. 

This pull request is just the first example of the proposition.

@repeatedly What do you think? Yes, each plugin author should support file permission check in #configure method of each plugin when we decide as this is an official way. 

PS. @repeatedly Could you tell me if you have a right place to put the file permission check utility function?