Breaking changes

Hi @cpursley

I tested version 0.3.0 but I still noticed breaking changes to version 0.1.6.

Version 0.1.6:

```elixir
> Html2Markdown.convert("Emphasis normal bold")
"*Emphasis* normal **bold**"
```

Version 0.3.0:

```elixir
> Html2Markdown.convert("Emphasis normal bold")
"*Emphasis*\n\nnormal\n\n**bold**"
```

This seems to be also happening with other tags like `span` or heading tags.