Make mod installer

[sanderd17]

It should be possible to install mods by loading a single file in the mod manager:

• The modder makes a file available for download, named something like modName.pyromod (containing the mod directory or zip, and the mod.json)
• The player downloads the file to his downloads
• From the mod-manager GUI, the player navigates to the file, and opens it
• The contents of the mod gets copied to the right location, and appears as an installed mod (with the option to enable it).