Adding multiple rules

[domchristie]

Make it easier to add multiple rules e.g.:

turndownService.addRules({
  table: { filter: '…', replacement: function () {…} },
  del: { filter: '…', replacement: function () {…} }
})

and:

turndownService.addRules([
  ['table', { filter: '…', replacement: function () {…} }],
  ['del', { filter: '…', replacement: function () {…} }]
])