Add OfflineSourceFolder parameter for offline installation

[rprieto]

This pull-requests implements an offline installation for Scoop. I documented the usage in the updated README.

1. Pre-download the following files into a folder of your choice

https://github.com/ScoopInstaller/Scoop/archive/master.zip as ScoopInstaller-Scoop.zip https://github.com/ScoopInstaller/Main/archive/master.zip as ScoopInstaller-Main.zip

2. Later, run

# From a local folder
.\install.ps1 -OfflineSourceFolder 'C:\Local\Path\To\Zip\Files'

# From a network path
.\install.ps1 -OfflineSourceFolder '\\UNC\Path\To\Zip\Files'

The files are not deleted, which means they can be re-used for other installations. This should address https://github.com/ScoopInstaller/Scoop/issues/3505.