Add vi like navigation

[daniel-vera-g]

After doing <Ctrl> + <Space> you can navigate through the results with the up/down keys.
It would be great to use the j/k keys. fzffor example uses <Ctrl> + P & <Ctrl> + N for navigation. This is very intuitive.

[weilbith]

Yes, it would be awesome to configure these key-bindings. So <Ctrl> + j/<Ctrl> + k are also quite popular. It's quite annoying to move the hands so far to the arrow keys. 🙊 Not sure how this is working here. 🤔

[bitsadmin]

Added support for Ctrl+J/K and submitted pull request to @pindexis.

https://github.com/pindexis/marker/pull/71