Show help when no input is given

[saint1991]

when no input is given, show help as follows,

$ sqlfmt
usage: sqlfmt [flags] [path ...]
  -d    display diffs instead of rewriting files
  -distance int
        write the distance from the edge to the begin of SQL statements
  -l    list files whose formatting differs from goreturns's
  -w    write result to (source) file instead of stdout

it also works fine when pipe input is given

$ cat sqlfmt/testdata/testing_gofile.go | sqlfmt
package sqlfmt

import (
        "database/sql"
)

func sendSQL() int {
        var id int
        var db *sql.DB
        db.QueryRow(`
SELECT
  any (
    SELECT
      xxx
    FROM xxx
  )
FROM xxx
WHERE xxx
LIMIT xxx`).Scan(&id)
        return id
}