Enable use of formatter contentSlug

[voxpelli]

The options forwarded to the formatter should enable one to make use of the contentSlug option (see https://github.com/voxpelli/node-format-microformat#options):

webpage-micropub-to-github/lib/handler.js

Lines 68 to 76 in 59e3b68

	.then(options => new MicropubFormatter({
	relativeTo: siteUrl,
	deriveCategory: categoryDeriver,
	deriveLanguages: options.deriveLanguages,
	permalinkStyle: options.permalinkStyle,
	filenameStyle: options.filenameStyle,
	filesStyle: options.mediaFilesStyle,
	layoutName: options.layoutName
	}))

That should solve juandpinto/node-format-microformat@bf9ad7b#commitcomment-31828341 and allow eg. a post like https://voxpelli.com/social/2018/12/40669/ / https://github.com/voxpelli/voxpelli.github.com/blob/master/_posts/2018-12-16-40669.md to have a content based slug

Ping @juandpinto