Give user more control on number of rows to be rendered at a time

[asheshv]

Allow user to select the number rows to be rendered in a block, and total number blocks to be rendered at a time.

This customisation will allow a user to define these parameters during initialisation based on the height of the tree container.

[coveralls]

Coverage remained the same at 67.414% when pulling b8d4e6c on asheshv:CLUSTERING into 7bf037e on cheton:master.