Add Unstable flag to transient

[johnhamelink]

This PR adds the ability to define flags which are passed to the just
executable itself. This is needed if, for example, a justfile requires
the --unstable flag.

[psibi]

Can you look into why tests are failing ? Also, can you add a test for this ?

[johnhamelink]

@psibi yes, certainly! I should have a chance have a look this week at some point.

[johnhamelink]

Hi, after some more reading and understanding of the code, I realised I didn't need to reinvent the wheel - I could make use of transient's saving feature to get the results I wanted, so long as transient arguments would also be applied to the list function. I've tested it thoroughly and couldn't trigger any issue with this approach.

[psibi]

@johnhamelink I guess we can close this PR then? Do you think we can document what you did via transient in the README.org ?

[johnhamelink]

@psibi I force-pushed some changes to this repo which I think are still valid, but yes I think we should link to this section of the transient docs: https://www.gnu.org/software/emacs//manual/html_node/transient/Saving-Values.html

[psibi]

Thank you!