feature: add --KeepMissing option to chkbit update command.

[varmint708]

This feature adds support for not deleting missing files or directories from the index (default is to remove them). This is for cases where the mount point might to temprary offline and you don't want to reindex them again when it is online and are using SkipExisting features, thus saves a lot of time if the repo exceeds a couple of terrabyte for those specific directores/files.

[laktak]

I still think you should have your index on the disk (#33) ;)

In any case the PR looks good but could you please change it to:

KeepMissing bool `help:"keep missing files in the index"`

The parser automatically adds a [no-] inverse in front and this looks better.

[varmint708]

Thanks @laktak for considering this feature. I really tried to get it working as you mentioned it in #33, but even after spending quite some time on it, it failed in some weird way due to number of disks, offline schedules for them or the way the files are spread on them and thus decided to add this. I hope it is ok.

[laktak]

Looks great now, thanks! Take care :)