Interface for developers for easier Grist instantiation.

[berhalak]

Adding a few UI components as a developers friendly abstraction over bootstrapGrist.

• [data-grist-csv/doc-open] attribute that can be added to any element. When clicked, it will open Grist preview in a popup window
• custom web component, that makes is easier to embed .grist or .csv documents on a page
• new global function previewInGrist that bootstraps Grist in a popup
• a new option initialContent that can hold csv content as a string.