This commit adds an improved reload code for Oh My Fish, besides
saving the history now the reloading technique keeps directory
history and stack and clears fish_greeting, for a transparent
transition.

The reloading code is now safe regarding to background jobs. exec
wipes fish job control, so the user-facing code under the (just-
introduced by this commit) `omf reload` command is kept safe by a
warning. For testing purposes, `omf update` and `omf remove` rolls
automatic refresh only when `OMF_AUTO_RELOAD` variable is set.

Recap of the commit:
- Add improved reload code (omf.reload)
- Add a safe reload code (omf.cli.reload)
- Add `omf reload` command
- Add opt-in reload to `omf update` and `omf remove` commands