Adding options to set header and footer in PDF generation

If the user wants to add a header or footer in each page of a generated PDF, he will be able to configure `book.json` like the following:
```
"pdf": {
	"pageNumbers": true,
	"paperSize": "a5",
	"headerTemplate": "<p><span>My Book</span><span>_SECTION_</span></p>",
	"footerTemplate": "<p>_PAGENUM_</p>"
}
```

The `ebook-convert` call will have the `--pdf-header-template` and `--pdf-footer-template` arguments if needed.

The user will be able to use Calibre's variables: `_PAGENUM_`, `_TITLE_`, `_AUTHOR_` and `_SECTION_`.