Introduce function to purge invalid entries from history

[Biont]

Fix #50

Introduces a public function that iterates over the history file and runs each line against a validation callback (referred to as purge_cmd when writing custom providers)

If this command exits with 0, the entry is kept in the file exits with 43, the entry is removed from file.

[WhyNotHugo]

Works fine for me.

If this command exits with 0, the entry is kept in the file. Any other exit status causes the removal of the entry.

Maybe do these the other way around? Otherwise, any unexpected internal error in the provider results in data deletion. Best to err on the safe side.

[PedroHLC]

This works well after deleting some files from ~/.local/share/applications. Are you still not considering pushing it upstream?

[Biont]

@WhyNotHugo I think your comment made a lot of sense and I've taken a fair share of time to consider its implications. Maybe it shouldn't have taken me half a year, but I'm slow at times.
So I had a look at common exit codes and chose 43 EIDRM (Identifier removed). I'm stretching its intended use a bit because as far as I understand, it's meant to be used in IPC, but then again, it's probably obscure enough to act as a very explicit and deliberate signal for removal.

What do you think?

(cc @PedroHLC, I hope we're getting close now :) )