Provide --single-instance or -d CLI parameter

[trajano]

I know there's a capability for single instance but it would be nice to have it as a cli-option since if I use the alias

alias gitk="gittyup &"

it adds noise to the terminal and it is also not portable so if I use aliae I have to create a specific one for Windows powershell

 gitk
[1] 37267
  trajano@noriko   iblum-stack    at/temporalize ≢  ?10 ~1   ead9d42f                                                       1.0.0 iblum-stack  21.0  21.7
 gitk
[2] 37275
[2]+  Done                    gittyup
  trajano@noriko   iblum-stack    at/temporalize ≢  ?10 ~1   ead9d42f                                                       1.0.0 iblum-stack  21.0  21.7