Option to populate a cache directory

[LPGhatguy]

Some consumers of Tarmac, like Roblox, want to have local copies of each uploaded asset, indexed by asset ID.

Tarmac should support an additional flag or separate command to create an output cache out of whatever the current state is by downloading images from Roblox:

# As a flag
tarmac sync --target roblox --cache-dir my-cache/

# As a separate command
tarmac sync --target roblox
tarmac make-cache my-cache/

[LPGhatguy]

Currently implemented as a property on the project file, but possible it should be turned into a flag for practical usage.

[LPGhatguy]

Done as the create-cache-map command, which rolled up two commands into one.