Add the ability to show content of a selected file

[loresuso]

What would you like to be added:
I would like to inspect also the content of the files being added. Probably the option is already there?

Why is this needed:
Might be useful for debugging purposes.

Additional context:

[wagoodman]

Once #569 is implemented this will be more easily implemented (it has a content API without needing to create a container from an image reference). Seems related to #107 (but not a duplicate), another thing that is needed here is to settle on the final direction for how the interaction would be shown (i.e. hit enter on a selected file and add another right-sided pane to the UI? replace the existing UI? etc).

[loresuso]

Nice! Why don't use less as a child process as a pager? We can make use of it (including search functionalities) and on exit, resume the initial UI.
Of course, depending on the platform, we can have a number of decent pagers, and maybe the user can also express which one to use using an environment variable if he does not like the default one.