Logging support and `--verbose` option

[patrickfreed]

swiftly currently prints some basic amount of human readable output, but we should also introduce the ability to log output using swift-log. We should also support a --verbose option to control how much output is included.

[cmcgee1024]

The verbose option is coming with #177 so that the default level is just to provide the high level actions that swiftly is taking, not the gpg messages, installer, etc. Choosing verbose would display all of those. Full logging might include the individual system commands that are executed, exit status codes, and other details.

[cmcgee1024]

These subcommands don't yet use the verbose flag at all:

• Proxy (probably not relevant, since the command-line arguments are for the destination binary, not swiftly itself)
• ListAvailable
• List
• Unlink
• Run
• Use