allow disabling log file output and creation of the `results` directory

[ZZiigguurraatt]

Upon startup, chantools tries to create a directory $PWD/results, and then writes a file chantools.log inside of it. If $PWD is currently read only, we get an error:

tester@fc610f97347b:/$ chantools help
panic: failed to create log directory: mkdir ./results/: permission denied

goroutine 1 [running]:
main.setupLogging()
	github.com/lightninglabs/chantools/cmd/chantools/root.go:315 +0x185
main.init.func1(0xc0001d1d00?, {0x1a16674?, 0x4?, 0x1a16678?})
	github.com/lightninglabs/chantools/cmd/chantools/root.go:78 +0xd5
github.com/spf13/cobra.(*Command).execute(0xc000224b08, {0x2bd8560, 0x0, 0x0})
	github.com/spf13/[email protected]/command.go:836 +0x59b
github.com/spf13/cobra.(*Command).ExecuteC(0x2b5cf00)
	github.com/spf13/[email protected]/command.go:960 +0x389
github.com/spf13/cobra.(*Command).Execute(...)
	github.com/spf13/[email protected]/command.go:897
main.main()
	github.com/lightninglabs/chantools/cmd/chantools/root.go:139 +0x55e
tester@fc610f97347b:/$ 

We need a command line option to disable log file output and creation of the results directory so that chantools can be used on read only filesystems on stateless devices.