#79 data URIs in HTML images

[Alistair Cloete]

This patch adds support for data URIs in HTML <img> tags, see
https://en.wikipedia.org/wiki/Data_URI_scheme, e.g.:

$pdf->writeHTML('<img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUA
  AAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO
  9TXL0Y4OHwAAAABJRU5ErkJggg==" alt="Red dot" />');

Unlike TCPDF's built-in image data strings, HTML files with data URIs can be previewed in a browser.
It's also possible to inline SVG images with data URIs, since the image type is part of the URI.