Add base HTML processor to replace tag for given xpath
Use have(n).items matcher for NodeReplacer spec
Describe HTMLProcessor::NodeReplacer.process

Use RSpec sugar to correctly describe behaviour in a way that generates
good documentation:

```
HTMLProcessor::NodeReplacer.process replaces the target XPath with the new tag
  0.15662 seconds ./spec/lib/html_processors/node_replacer_spec.rb:18
```