Omit horizontal decoration lines if empty

This allows for more flexible compact layouts: even the compact mode can
be redefined in terms of 'chars' settings by using the empty string for all the
horizontal 'mid' characters.

By using the following configuration:
 var table = new Table({
   chars: {
       'mid': ''
     , 'left-mid': ''
     , 'mid-mid': ''
     , 'right-mid': ''
   },
   style: {
       head: []
     , border: []
   }
 });

it's now possible to get results like this (note the lack of the horizontal
line between rows):
 ┌────────────┬─────┬──────┐
 │ foo        │ bar │ baz  │
 │ frobnicate │ bar │ quuz │
 └────────────┴─────┴──────┘

By setting all chars to empty with the exception of 'middle' being set to a
single space and by setting padding to zero, it's possible to get the
most compact layout with no decorations:
 foo        bar baz
 frobnicate bar quuz