Same as the feature in maidr js

Feature Request

Motivation

Users want to be able to save a spot, and quickly nav back to it or at least hear it again.

Proposed Solution

• Mark a spot with M1 (shift m, then 1), can use 0-9 to have 10 marks
• Play a marked spot with m1 (m, then 1), which does the same thing as Space
• Jump to a marked spot with j1 (j, then 1)

Also add keys to help menu, and a "No mark 1 exists" etc if user does m1 without having done M1.