Tag command & Horizontal labels rotation

#134 -> Thanks to @cgddrd, labels properly rotate in horizontal mode. You can provide a labelRotation configuration to your template too:

var graphConfig = new GitGraph.Template({
    branch: {
        color: "#000000",
        mergeStyle: "straight",
        labelRotation: 0 // -> can be configured
    },
});

var gitGraph = new GitGraph({
  template: graphConfig,
  mode: "extended",
  orientation: "horizontal"
});

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#75 -> Now you can tag current HEAD with .tag( "my-tag" ). You can still pass tag options to the method:

var gitGraph = new GitGraph({ template: "metro" });

gitGraph
  .commit("Tag this commit")
  .tag("my-tag");

// alternatively
gitGraph
  .commit("Tag this commit")
  .tag({
    tag: "my-tag",
    tagColor: "green",
    tagFont: "normal 14pt Arial",
    displayTagBox: false
  });

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#121 -> Fix rendering when parent branch has no commit.