Add vim's 'ctrl+d','ctrl+u','g g', and 'G' actions

* 'd' - scroll down half page
* 'u' - scroll up half page
* 'g g' - scroll to top of page
* 'G' - scroll to bottom of page
Merge pull request #12 from woahdae/more_actions

Add vim's 'ctrl+d','ctrl+u','g g', and 'G' actions