Adding offset as an optional way to paginate

[louisnow]

Offset would be useful feature similar to mongoose-paginate.
We can add it in a non breaking way like so.

if(options.hasOwnProperty('offset'))  {
// perform skip/offset logic
} else {
// default behaviour
}

Adding this feature could make certain implementations that use the library and have skip but not page in their options not work as they used to. (which shouldn't be our concern but just a heads up in the changelog maybe)

[louisnow]

I chose to rename the title from skip feature to offset because mongoose paginate does so. However, I see no other reason to use the word offset instead of skip as mongoose uses skip and compatibility with that is more useful.