Add functionality to have a local providers file

It is occasionally useful to be able to directly link to a feed from an application, instead of having to pass by a remote server to resolve the provider information, and this code allows for such a thing to be achieved, by adding an option to knsrc which allows for adding in a providers file next to the knsrc file.

The patch further cleans up the XmlLoader class a touch, as a result of adding support for non-remote file locations.

Copy of the new documentation:

To use a local providers file instead of a remote one (this is useful for example in cases where you wish to use an OPDS feed, which likely does not have a providers file, and you lack a server on which to host one of your own), you can set UseLocalProvidersFile=true instead of ProvidersUrl. This will make the engine attempt to load a file located next to your knsrc file, with the same name as that file, with .providers appended. For example, if your configuration file is called "appname.knsrc", then your providers file should be named "appname.providers"