Add indent method to Thor::Shell::Basic

__Example:__

```ruby
say "Hello!"
indent do
  say "I'm indented 2 spaces!"
  indent do
    say "I'm indented 4 spaces!"
    indent(2) { say "I'm indented 8 spaces!"  }
  end
end
say "I'm not indented anymore!"
```

__Output:__
```
Hello!
  I'm indented 2 spaces!
    I'm indented 4 spaces!
        I'm indented 8 spaces!
I'm not indented anymore!
```