Use improved string formatting

Use string formatting by name instead of formatting by position when
creating the file description.

 - adds the method `formatString` to `MediaWikiUtils`
 - modifies `MediaWikiInterface` to use `MediaWikiUtils.formatString`

See issue [#29: Move file description template into a file and improve
the string formatting](https://github.com/ireas/LrMediaWiki/issues/29).