Make new `rifle` behavior backwards compatible.

There's 4 possible situation:

  1. `VISUAL` set, `EDITOR` set
  2. `VISUAL` set, `EDITOR` not set
  3. `VISUAL` not set, `EDITOR` set
  4. `VISUAL` not set, `EDITOR` not set

All but _No.2_ are already properly handled.
Behavior can differ depending on whether you have an old or a new
`rifle.conf` in cases _No.1_ (if the variables differ the new approach
prefers `VISUAL` instead of `EDITOR`) and _No.2_ (new approach will use
`VISUAL` whereas the old approach would've fallen back to _vim_).
_No.3_ is likely the most common case and works almost identically with
either approach.
_No.4_ still falls back to _vim_ as before.

Fix #1048

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