render images with spaces in path

[katrinleinweber]

continuation of Code52/DownmarkerWPF#405

Please consider adding support for spaces in folder and filenames. My use case for example is storing images and figure in sub-folders of the working directory, an syncing it with colleagues that sometimes use Mac OS X. MacDown has no problem with spaces for example, so that renaming, copying and/or reordering the image files is not a feasible option.