Add option to save map images as JPEGs rather than PNG

[burkmarr]

Email from Andy Amphlett:

Saving maps via the composer image using batch map mode. Maps are saved as png files which are very large. GGIS can save high quality jpegs at c. 1/8th the file size of the png files. I have not found a way to get the plugin to save files as jpegs. My solution was to import the png files into Adobe Lightroom and save as jpegs there. Again, not a big problem but it would be good if there was a way to avoid this extra step.