support standalone home-manager specialisations

[daschw]

Hi thanks for this awesome helper!
I have dark and light theme specialisations in my home-manager config and I would like to pass a specialisation arg to nh home switch just like for nh os switch. Furthermore I'd like to be able to make nh home switch auto-detect and activate the current specialisation in my home-manager config.
I saw in #80 that this is currently not supported. So I tried to implement this myself following along the implementation for the nixos system. This PR is what I came up with. I tested the ~/.local/share/home-manager/specialisation part with my personal home-manager configuration. However, I can change the path, if a different location is preferred.
I suppose this fixes #80.

[viperML]

Thank you, if you just copied the NixOS implementation then it looks good to me.

I don't have much time to test this (as I neither use HM specialisations), and we also don't have any type of test suite, so it would be nice to hear from other people if this works for them.

[daschw]

Thanks for your quick response! Yes, I basically just copied the NixOS implementation. I completely understand that your time is limited. Is there anything I can do to make testing for you or others easier?

[viperML]

Is there anything I can do to make testing for you or others easier?

NH doesn't have any test, but I don't know what the best approach is to testing this type of application (I also don't have much experience with testing anyways).

But I think that is out of the scope of this PR.