Add indent option

indent: 2
``` rb
require "slop"

Slop.parse(help: true, indent: 2) do
  banner 'Usage: foo.rb [options]'

  on 'name=', 'Your name'
  on 'p', 'password', 'An optional password', argument: :optional
  on 'v', 'verbose', 'Enable verbose mode'
end
```

```
$ ruby slop-test.rb --help
Usage: foo.rb [options]
      --name          Your name
  -p, --password      An optional password
  -v, --verbose       Enable verbose mode
  -h, --help          Display this help message.
```

default
``` rb
require "slop"

Slop.parse(help: true) do
  banner 'Usage: foo.rb [options]'

  on 'name=', 'Your name'
  on 'p', 'password', 'An optional password', argument: :optional
  on 'v', 'verbose', 'Enable verbose mode'
end
```

```
$ ruby slop-test.rb --help
Usage: foo.rb [options]
        --name          Your name
    -p, --password      An optional password
    -v, --verbose       Enable verbose mode
    -h, --help          Display this help message.
```