Feature request: add shasum field

[priyadarshan]

Although it is not important in the git ecosphere, it would be nice to be able to specify an optional shasum field, to be used with the -f switch, especially if #3 is accepted and implemented.

It could perhaps leverage local unix shasum, as that is very fast.

Adding a shasum field would make git-store-meta a very powerful tool for many.

Thank you for considering this.