permit external URL resolution for images

[danlobo]

The actual code uses the url.openStream(), which doesn't give too much options to control how to obtain the image (from a local file, for example). This change allows the user to manipulate how to load URL images. The requirement is to create a subclass of URLResolver and its resolve method, returning the actual InputStream from the given url.

The URLResolver instance is found as a BrowserConfig's property.

[radkovo]

Many thanks for the PR. You are right that using url.openStream() is not extensible enough. But it seems to me that the URLResolver you propose duplicates the functionality of DocumentSource that is used for loading the remaining resources. It could be sufficient to adjust image loading to use DocumentSource as well. I have made a proposal in a separate imgload branch (see the last two commits). Does this solution meet your needs too?

[danlobo]

Looking better at DocumentSource, you're right, it's the right class for this task. Your code meets my needs perfectly! Thanks for the corrections.

[radkovo]

Ok, I merged imgload to master and I am closing this PR.