Add optional "lookupHeader" to replace default header

[Dominic-Preap]

Can we have optional "lookupHeader" to replace default header "Accept-Language"?

i18next
      .use(FilesystemBackend)
      .use(i18nextMiddleware.LanguageDetector)
      .init({
        detection: {
          order: ['header', 'querystring'],
          lookupQuerystring: 'lng',
          lookupHeader: 'x-language'
        },       
      });

[jamuhl]

feel free to provide a PR taking value from options: https://github.com/i18next/i18next-express-middleware/blob/master/src/languageLookups/header.js#L12 and default to the "Accept-Language" if not set

[Dominic-Preap]

@jamuhl would you be able to review my PR?

[Dominic-Preap]

@jamuhl Would you mind update this option in README.md? Thanks.

[jamuhl]

@Dominic-Preap was added to readme - thank you for the reminder...