Update index.html
Keyboard arrows to change the value inside parentheses

Keyboard up/down to increase/decrease value that is inside parentheses
containing browser caret.

By default, delta is 1 for integers and 0.1 for floats.
Use `shift` for x10, `ctrl` for x0.1.

Tested in Firefox and Chrome.
merged in Jakub's changes to allow arrow up/down to increase/decrease values