Add Vim-inspired Ctrl-E/Ctrl-Y line scrolling in article pane

[ivanpustogarov]

This adds Vim-inspired keyboard shortcuts for line-wise scrolling in the article pane.

• Ctrl-E scrolls the article pane down by one line
• Ctrl-Y scrolls the article pane up by one line

The implementation follows the existing keyboard handling path in AppController.

• Vim-style navigation keys make it easier to read articles from the keyboard:
  1. Use (already existing) Up/Down (or Karabiner-remapped k/j) to choose prev/next article.
  2. Use Ctrl-E/Ctrl-Y to quickly scroll the article body.

[barijaona]

I am not comfortable having this behavior in main tab and not in other tabs.
Opinions @ViennaRSS/maintainers ?

If we decide to go this way, updating keyboard.html in help file will also be needed.