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