Previewer, dialog and illustrator UIs.

[dharmx]

Implement a previewer for viewing metadata and file contents. As for pad, implement a general floating window previewer. Like if the current line is a terminal command then ask if one should run the command.

Also, allow a way for the users to write their own illustrator.

Note

Reuse the illustrator UI (buffer) for telescope previewers as well!
No need to write a separate preview maker for the telescope pickers.