Ability to grab images directly from field content

Issue Summary

I wanted to use this module in conjunction with the salesforce_feeds module, to allow images to be pulled directly out of a field content from a salesforce object. I have patched the module to allow this, but currently it's a little hardcoded to my specific usage as I just needed to get it one quickly.

Basically, the idea is to add an additional setting on the feed field specifying of the content of that feed refers to a webpage (which is what the module assumes currently) or if it actually contains HTML content (as in the case of these salesforce fields). If the field contains a URL, then the module acts as it does now (fetching the content of that url and scanning it for images). Alternatively, if the field contains raw html content then that content is scanned for images.

I'll upload a patch once I have cleaned it up a little.