[#3] Implemented a command line interface for Elvis.

```bash
$ ./elvis --help
Usage: elvis [-h] [-c <config>] [--commands]

  -h, --help    Show this help information.
  -c, --config  Provide the path to the configuration file.
  --commands    Show available commands.
$ ./elvis --commands
Elvis will do the following things for you when asked nicely:

rock             Rock your socks off by running all rules to your source files.
```