Use Left / Right arrows to enter and leave directories

[certik]

Is your feature request related to a problem? Please describe.
I would expect the Right arrow to enter a directory, and Left arrow to leave it. Instead the arrows have a weird behavior as reported in #111, and it was fixed to edit input.

Describe the solution you'd like

Rather, I propose to fix it to enter and leave directories.

[Canop]

I'm making it possible to override those keys and allow this in the configuration:

[[verbs]]
key = "left"
execution = ":parent"

[[verbs]]
key = "right"
execution = ":focus"

Side note: I'd be interested in special configuration collections, with rationales, in order to propose them on the site.