Add flag to store retronet files into storage

[sammargh]

The current setup for handling retronet files while works introduces problems with some of the cloud features within retronet.

While running cloud cp/m via the homebrew list the read/write C: disk never will be stored on the computer and is destroyed as it is only in memory. Currently the only workaround is to store c.dsk as C.DSK in the storage location defined on the connection. As you had suggested a good workaround is to include a variable in the config to flag files to be stored persistently:

"PersistentCache": "*.dsk"
"PersistentCache": "c.dsk"

[thorpej]

I still haven't decided how I want to do this yet, so I'm gong to cut 1.2 without resolving this issue, but will work on it over the coming week.