transmission: add extraFlags configuration option
transmission: add extraFlags configuration option

Makes service more customizeable and makes debuggingin easier through
the use of flags like `--log-debug` or `--dump-settings`.

Signed-off-by: Jakub Sokołowski <1d3764b91b902f6b45836e2498da81fe35caf6d6@status.im>

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs