Add flexible minute and hourly rotation frequency

For example: 
You can pass in config options frequency: '5m' or frequency: '\<number\>m' etc to have a \<number\>minute(s) rotation.
In the same way you can pass in frequency: '1h' or frequency: '\<number\>h'  etc to have a \<number\>hour(s) rotation.

Updated README.md with the details of the new functionality.

Current functionality remains unchanged so it will not effect the way this lib is used currently.