Allow important paths to be configured during Windows installation

I suspect Linux users may consider this a minor issue, because drive mounting is so flexible, but Windows users expect to be able to pick important paths during installation. Sometimes they are particular about the path because they want to use a non-default drive (e.g. because C: might be a relatively-small SSD). Two important paths that should be configurable during installation are:

1. The path specified in the STACK_ROOT environment variable. (There's already an issue for this: #1443.)
2. The path specified in the 'local-programs-path' setting of the %STACK_ROOT%\config.yaml.

Regarding the second one, the config.yaml file doesn't generally even exist until you've run stack at least once. Doing this will download files to the default local-programs-path, which is %LOCALAPPDATA%\Programs\stack. The only way around this, currently, is to already know up-front to manually create a c:\sr folder, add a config.yaml to it, and provide that setting within the file. That's not a reasonable thing to require of a new Windows user of stack.