Implement safe shell reloading inside omf plugin

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`, and usages under
`omf remove` and `omf update` are now kept safe by a warning.

Recap of the commit:
- Add improved reload code (omf.reload)
- Add a safe reload code (omf.cli.reload)
- Add `omf reload` command