Raise an error when index name isn't a string

Rails's `add_index` takes an option hash as last argument instead of an index name, so people sometimes do this:

```ruby
Lhm.change_table :foo do |m|
  m.add_index [:bar], name: 'name'
end
```

excepting an index named `name` to be created, but it actually ends up with the name `{:name=>"name"}`.

Since changing the signature of `add_index` is a breaking change, I've instead made it raise an exception when the index isn't a string or symbol.

@arthurnn