Support non-HTML templates

Static Marks supports custom templates. The logic is not flexible, though. It depends on the way the default template (Static Marks App) works: bookmarks are [injected as plain text](https://github.com/darekkay/static-marks/blob/196ee89255058260bd376e33caeafd8db193928a/src/render.js#L9) and require the template to use JavaScript to access them.

The logic should be adjusted to support transforming bookmarks into any other format. This will also open up the project and simplify migrating to other projects. An opinionated template language (e.g. ejb, handlebars or nunjucks) can be predefined.

Related: #44