Add keyboard shortcuts

Keyboard shortcus would be a nice touch to add client-side as it would alow the bypassing of buttons

If this gets merged before #18, I'll make changes to #18's PR to include keyboard shortcuts for adding time.

### Proposed Shortcuts
- <kbd>s</kbd> - Toggle timer
- <kbd>a</kbd> - Focus add time input box
- <kbd>S</kbd> - Focus set time input box