add a `help` command, document `run` in the readme

go/linguaevalia/README.md

[one for rust](./bin/run-rust)) or you can implement the `Language`
interface.
## Additional commands
`linguaevalia` can also be used to run programs directly, instead of using the
server:
```
$ linguaevalia run hello-world.rb
Hello, World!
# or alternatively:
$ cat hello-world.rb | linguaevalia run -l ruby
Hello, World!
```
When passing code to the `run` command via stdin, you *must* set the language
to be used using the `-l` flag as well. If a file name is passed, the type is
inferred from the extension, but you can override it with `-l` if you want.
## Development setup
```

go/linguaevalia/linguaevalia.go

	}
}
func runHelp() {
	fmt.Printf(`Usage: %s [cmd] [options]
Availlable commands:
server		- Starts a web server. (Default.)
run		- Runs code from a file or from stdin.
		  (If running from stdin, you must pass the language
		   using the -l flag.)
help		- Display this help message.
`,
		os.Args[0])
}
func parseCommand() (string, []string) {
	if len(os.Args) == 1 {
		return "server", []string{}
		runServer()
	case "run":
		runOnce(flag.Args())
	case "help":
		runHelp()
	default:
		fmt.Println("Error: Unknown command:", cmd)
		os.Exit(1)