Support for specifying the lifetime with ssh-add

[cargocultprogramming]

This adds the option to add a lifetime argument so that the maximum lifetime of keys can be limited via ssh-add. Branched from #34

[BrechtSerckx]

What's the status of this?

[cargocultprogramming]

What's the status of this?

Moved it forward to master. Works for me.

[JokerQyou]

Would it be feasible to read timeout value (and other ssh-add option values) from the vault fields of the secure note? For example to add a field with name -t and value 30m, and the script automatically apply the option -t 30m to ssh-add execution. Ideally this means all options supported by ssh-add could be controlled at per-key level.

[cargocultprogramming]

Should be possible.

However command line arguments would be an issue. What has precedence? Needs a bit of thought I guess.