Create test suite

I added a test suite! If you were going to do emphasis next (`*text*` to `<em>text</em>`), you might add something like this to `spec/converter_spec.rb` to test it.

```ruby
it "emphasises text using <em> tags" do
  expect(converter.convert_paragraph("one *two* three")).to eq "<p>one <em>two</em> three</p>"
end
```

Running all the tests in the terminal looks like this.

```bash
$ bundle exec rspec
.

Finished in 0.00095 seconds (files took 0.10986 seconds to load)
1 example, 0 failures
```

It takes it 0.00095 seconds to tell you if everything's working perfectly!