feat: add rows count

[ajunior]

Add a public method to count the number of rows in the table.

Example:

numberOfRows := t.RowCount()
if numberOfRows == len(arguments) {
    // do something
}

This PR refers to the issue #30.

[ajunior]

@owenrumney would you please check it?

[owenrumney]

LGTM - thanks