feat: add method to clear the table

[ajunior]

Add a method to clear the table. It does not clear the header.

Example:

t := table.New(os.Stdout)
t.SetHeaders("A", "B", "C")
t.AddRow("1", "2", "3")
t.AddRow("4", "5", "6")
t.Clear()

This PR refers to the issue #25.

[owenrumney]

LGTM, thanks

[ajunior]

@owenrumney can you please release this feature?

[owenrumney]

@ajunior - apologies, not sure how I approved then didn't merge and release. Done now