Circumvent exception for explain without file
Circumvent exception for explain without file

Currently (0.4.3) `mix credo explain` will fail because of a missing function clause. Running `mix credo explain --help` suggests that files are optional ( `[path_line_no_column]` ). I'm not sure what future plans are, but to prevent this error from showing up, this fix prints help in case of no supplied files, with a slightly different help message to prevent confusion.

Just FYI, this is the current error (first part):
```
$ mix credo explain
** (FunctionClauseError) no function clause matching in Credo.CLI.Command.Explain.run/2
    lib/credo/cli/command/explain.ex:15: Credo.CLI.Command.Explain.run([], %Credo.Config{all: false, check_for_updates: true, checks: [{Credo.Check.Consistency.ExceptionNames}, ... etc .. help: false })
    lib/credo/cli.ex:58: Credo.CLI.main/1
    (mix) lib/mix/cli.ex:58: Mix.CLI.run_task/2
    (elixir) lib/code.ex:363: Code.require_file/2
```
rrrene/credo,rrrene/credo