Remove column separators in tables

As shown in the table demo, there is a method to remove row separators:
```
ui.Render(primary)

table2 := widgets.NewTable()
table2.Rows = [][]string{
[]string{"header1", "header2", "header3"},
[]string{"Foundations", "Go-lang is so cool", "Im working on Ruby"},
[]string{"2016", "11", "11"},
}
table2.TextStyle = ui.NewStyle(ui.ColorWhite)
table2.TextAlignment = ui.AlignCenter
table2.RowSeparator = false
table2.SetRect(0, 10, 20, 20)

ui.Render(table2)
```

The table widget seems very promising to display things as a grid (for example I am attempting to make my own terminal based battleship). Only not having the option to hide column separators makes it impossible to do so...

Start from the widgets.NewTable implementation and follow the existing RowSeparator option shown in the table demo. Check how table borders are rendered; done means column separators can be disabled independently while the existing row-separator behavior still works.

Written by the indexing model from the issue text.