Move PrintDefaults() to flag.Usage for '-h', '--help' option.

In original code, command line flag information is not displayed when `-h` or `--help`
flag is specified as below.

```
% migemogrep -h
migemogrep v0.1.0

Usage: migemogrep [options] pattern [files...
```

After applying this patch

```
% migemogrep -h
migemogrep v0.1.0

Usage: migemogrep [options] pattern [files...]
  -d="": Alternate location to dictionary
  -n=false: print line number with output lines
```