configurable filenames

[manolomartinez]

• Documenting the new feature
• Read the new config option 'downloaded_filename' and use it to construct the fullpath
• It's filename that we care about now, not placeholders.filename
• substitute should be a method of Placeholders
• Use the new Placeholders.substitute() method

[Simon818]

Getting back to this after some time. I think this looks great. One nice touch would be to add a {file_extension} placeholder that pulls the extension off placeholders.filename. This allows defining a download filename without having to manually add the extension to the end. It is also theoretically possible for podcasts to have files with multiple extensions, so in this case any custom filename that hardcodes the extension is not going to work. I realize this is a bit of a hack, so thoughts are welcome.