Add a mod update checker

[EliteMasterEric]

This would add a utility function that performs the following steps:

• Polymod fetches the metadata of each installed mod.
• Polymod retrieves a field from the metadata, which contains a URL.
• Polymod queries that URL, which provides, for each API version of the program, the latest compatible mod version along with a download link for that version.
• Polymod would then provide a list of mods that have updates, along with some metadata about the mod and a download link for the update.

There would then be another utility function which would allow Polymod to download a mod and place it in the mods folder where it can be accessed.