By convention exit(1) is reserved for general errors, while exit(2)
means command misuse. Stdlib `flags` package invokes `os.Exit(2)`
after printing out the usage message. This commit changes the empty
files list exit code to be aligned with this behavior.
Currently executing `boltbrowser --help` creates a file with name
`--help` in the current working directory, which is counterintuitive.
This commit adds command-line flags support to boltbrowser and changes
it to print out the usage message instead.
Andrew Slotin