Add option to list all providers which are in the paste db

Right now, -Dq respects the provider (including if it has been set by -P).

However, this means there is no way to list all pastes in the paste db (only all pastes present in the current provider).

It would be nice (and good for future-proofing) if we had an option to list all of the providers in the paste db, so you could at least run the following to list all pastes:

$ for i in "$(pbpst -DH)"; do pbpst -Dq '<search>' -P "$i"; done

[x] Add -H|--providers under -D
[x] Update tab completion
[x] Update documentation
- [x] Include the above snippet in pbpst(1):Examples section

HalosGhost / pbpst

Add option to list all providers which are in the paste db #58