feature: Add label for img output node links

[kimitoboku]

I want to add information such as IP address to the output of graphviz.
And, just add mermaid output format that only graph connection.

Example:

nodes:
- name: client
  image: nicolaka/netshoot
  interfaces:
  - { name: net0, type: direct, args: R1#net0, label: "10.0.0.2/24"}
- name: R1
  image: nicolaka/netshoot
  interfaces:
  - { name: net0, type: direct, args: client#net0, label: "10.0.0.1/24"}
  - { name: net1, type: direct, args: root#net0 , label: "10.0.1.1/24"}
  - { name: net2, type: direct, args: resolver#net0 , label: "10.0.2.1/24"}
  - { name: net3, type: direct, args: nomns#net0, label: "10.0.3.1/24"}
- name: root
  image: nicolaka/netshoot
  interfaces:
  - { name: net0, type: direct, args: R1#net1, label: "10.0.1.2/24"}
- name: resolver
  image: nicolaka/netshoot
  interfaces:
  - { name: net0, type: direct, args: R1#net2, label: "10.0.2.2/24"}
- name: nomns
  image: nicolaka/netshoot
  interfaces:
    - { name: net0, type: direct, args: R1#net3, label: "10.0.3.2/24"}

digraph  {

	n2[label="R1"];
	n1[label="client"];
	n5[label="nomns"];
	n4[label="resolver"];
	n3[label="root"];
	n2->n3[arrowhead="none",fontsize="8",headlabel="net0(10.0.1.2/24)",labelfloat="true",taillabel="net1(10.0.1.1/24)"];
	n2->n4[arrowhead="none",fontsize="8",headlabel="net0(10.0.2.2/24)",labelfloat="true",taillabel="net2(10.0.2.1/24)"];
	n2->n5[arrowhead="none",fontsize="8",headlabel="net0(10.0.3.2/24)",labelfloat="true",taillabel="net3(10.0.3.1/24)"];
	n1->n2[arrowhead="none",fontsize="8",headlabel="net0(10.0.0.1/24)",labelfloat="true",taillabel="net0(10.0.0.2/24)"];

}

$ ./tinet img -c example.yaml  -f mermaid
graph TD;
	n2("R1");
	n1("client");
	n5("nomns");
	n4("resolver");
	n3("root");
	n2-->n3;
	n2-->n4;
	n2-->n5;
	n1-->n2;