Re-wrap documentation lines

The problem can be seen when running `wp scaffold taxonomy --help`:

Current output:

```
  [--theme]
    Create a file in the active theme directory, instead of sending to
STDOUT. Specify a theme with `--theme=<theme>` to have the file placed in that theme.
```

Expected output:

```
  [--theme]
    Create a file in the active theme directory, instead of sending to STDOUT.
    Specify a theme with `--theme=<theme>` to have the file placed in that theme.
```

And it seems the [wordwrap](http://php.net/manual/en/function.wordwrap.php) function will come in handy.