add customization and fix keybinding

Hello,

This fixes the `q` keybinding and all subsequent keybindings that may be added. The `if` in elisp is like a tertiary operator. If you don't use `progn`, the second line is considered the `else` clause. So it used to say: if there's a `hackernews-map` then add the `g` keybinding to it, otherwise add the `q` one. In fact, if you don't plan on having an `else` clause, I believe you can use `when` instead of `if`.

It also adds the possibility for users to customize the color of the links in the hackernews buffer. I have a light theme and `green` simply was unreadable.

Have a nice day!
Louis "Baboon" Kottmann