Add keyboard shortcuts.


| shortcout    | checkbox |
| ------------ | -------- |
| <kbd>o</kbd> | org                   |
| <kbd>i</kbd> | issues                |
| <kbd>r</kbd> | repositories          |
| <kbd>c</kbd> | comments              |
| <kbd>f</kbd> | forks                 |
| <kbd>e</kbd> | stars                 |
| <kbd>a</kbd> | Toggle all checkboxes |

<kbd>s</kbd> is GitHub's search function. So I switch to `e`.