We want to export some of our CLI code to re-use in other projects. But in Golang you cannot import code from a `main` package. So we need to move the actual code into its own package and only have the `func main()` in the `main` package.
- introduces new parser in `cmd/lncli/arg_parse.go` - converts start_time and end_time flags to strings - adds default value for end_time