Feature: Make shares self-contained

[MarcelWaldvogel]

The receivers of the shares might not know/remember what the threshold is and whether the shares belong together. How about having an extended form of the shares as follows?

<share>[/<threshold>[/identifier]]

The identifier could be specified on the split command line, with a small (32 bit?) randomly generated identifier as the default. This would allow the recipient to be able to distinguish his/her shares.

On combine, the threshold would be read from the first share, if not specified on the command line. Also, thresholds and identifiers, if present, would be compared to reduce the possibility for wrong combinations.

[itsabdelrahman]

@MarcelWaldvogel Outstanding proposition! Thanks a lot - will explore 👌