* README.md: Add to readme, under special section.
* features/insert-letters.feature: Add scenarios.
* features/step-definitions/multiple-cursors-steps.el: Add call.
* features/support/env.el: Add keybinding for insert-letters to H-3.
* mc-separate-operations.el: Add function and helper functions.
* multiple-cursors-core.el: Add insert-letters to functions that run once.
* multiple-cursors.el: Add to readme.
