Expose config for custom header & footer per page.

This brings the config object up to date with the dom api.

I guess this closes #154

```
  "header": {
    "height": "28mm",
    "contents": {
      "first": "first page",
      "2": "second page",
      "last": "last page"
    }
  },
  "footer": {
    "height": "28mm",
    "contents": {
      "first": "first page",
      "2": "second page",
      "last": "last page",
      "default": "Page {{page}} of {{pages}}"
    }
  }
```

Author: marcbachmann

README.md

@@ -61,7 +61,7 @@ pdf.create(html [, options], function(err, buffer){});
 
 ### Footers and Headers
 
-`html-pdf` can read the header or footer of a page out of the html source. You can either set a default header or footer or overwrite that by appending a page number (1 based index) to the `id="pageHeader"` attribute of a html tag.
+`html-pdf` can read the header or footer either out of the `footer` and `header` config object or out of the html source. You can either set a default header & footer or overwrite that by appending a page number (1 based index) to the `id="pageHeader"` attribute of a html tag.
 
 You can use any combination of those tags. The library tries to find any element, that contains the `pageHeader` or `pageFooter` id prefix.
 ```html
@@ -108,14 +108,19 @@ config = {
   },
   "footer": {
     "height": "28mm",
-    "contents": '<span style="color: #444;">{{page}}</span>/<span>{{pages}}</span>'
+    "contents": {
+      first: 'Cover page',
+      2: 'Second page' // Any page number is working. 1-based index
+      default: '<span style="color: #444;">{{page}}</span>/<span>{{pages}}</span>', // fallback value
+      last: 'Last Page'
+    }
   },
 
 
   // Rendering options
   "base": "file:///home/www/your-asset-path", // Base path that's used to load files (images, css, js) when they aren't referenced using a host
-  
-  // Zooming options
+
+  // Zooming option, can be used to scale images if `options.type` is not pdf
   "zoomFactor": "1", // default is 1
 
   // File options

lib/scripts/pdf_a4_portrait.js

@@ -144,16 +144,18 @@ function getContent (page) {
 // Creates page section
 // --------------------
 function createSection (section, content, options) {
+  options = options[section] || {}
   var c = content[section] || {}
-  var o = options[section] || {}
+  var o = options.contents
+  if (typeof o !== 'object') o = {default: o}
 
   return {
-    height: o.height,
+    height: options.height,
     contents: phantom.callback(function (pageNum, numPages) {
-      var html = c[pageNum]
-      if (pageNum === 1 && !html) html = c.first
-      if (pageNum === numPages && !html) html = c.last
-      return (html || c.default || o.contents || '')
+      var html = o[pageNum] || c[pageNum]
+      if (pageNum === 1 && !html) html = o.first || c.first
+      if (pageNum === numPages && !html) html = o.last || c.last
+      return (html || o.default || c.default || '')
         .replace('{{page}}', pageNum)
         .replace('{{pages}}', numPages) + content.styles
     })