zsh: support extra settings in oh-my-zsh plugins

[carlostome]

Hi,

I've added support to pass settings to plugins.

My use case is adding indentities to ssh-agent (oh-my-zsh/ssh-agent).

If there is a more direct way to achieve so, I'll be glad to hear.

[rycee]

I'm also not completely certain what the best solution is but this seems OK to me.

@dermetfan seems OK to you?

[dermetfan]

Could initExtraBeforeCompInit not be used for this? Though in order to only include the extra config when oh-my-zsh is enabled the user would have to check that on his own, like this:

programs.zsh.initExtraBeforeCompInit = lib.optionalString config.programs.zsh.oh-my-zsh.enable ''
  zstyle :omz:plugins:ssh-agent identities id_rsa id_rsa2 id_github
'';

I'm OK with this PR to make this use case more ergonomic, once @rycee's suggestions are applied.

[carlostome]

I applied your suggestions, you can squash and merge. Many thanks!