feat(table): add method to check if the table is empty

[ajunior]

Add the public method IsEmpty() to check if the table has no data.

Example:

t := table.New(os.Stdout)
t.SetHeaders("Key", "Value")
t.AddRow(key, value)

if t.IsEmpty() {
    fmt.Println("No records to show")
} else {
    t.Render()
}

This PR resolves the issue #28.

[CLAassistant]

All committers have signed the CLA.

[owenrumney]

LGTM, thanks @ajunior