Create `scribe.registerHtmlFormatter` interface
Rename `HTMLFormatter` to `HTMLFormatterFactory` to reduce confusion
Make `scribe.htmlFormatterFactory` private
Create `scribe.registerPlainTextFormatter` interface
Rename `Formatter` to `FormatterFactory` to reduce confusion
Make `scribe.plainTextFormatterFactory` private
Rename for consistency with DOM API cases
Merge pull request #141 from guardian/oja-tidy-up-3

Tidy up