Support embedding local images in generated PDFs

[Ocramius]

While working with the tool today, I noticed that Markdown containing local imports will not work:

![An Image](./file.png)

This is because we are missing two things:

1. --enable-local-file-access to allow file:// usage in WkHtmlToPdf
   • see https://github.com/wkhtmltopdf/wkhtmltopdf/issues/2660#issuecomment-663063752
   • see https://github.com/wkhtmltopdf/wkhtmltopdf/issues/4536 https://github.com/Roave/DocbookTool/blob/00942cc5d55f80734d4e4cbb46023fcc09cdb1e7/src/Writer/MultiplePdfFilesWriter.php#L54-L66
2. the relative files need to be dragged in the build workspace: currently, we copy just the HTML file to be rendered: https://github.com/Roave/DocbookTool/blob/00942cc5d55f80734d4e4cbb46023fcc09cdb1e7/src/Writer/MultiplePdfFilesWriter.php#L50-L52

This requires some redesign of the tool, so that the HTML files are in the same location as the sources (during builds), rather than in a temporary/separate location