Added ability to customize graph output

I enjoy this Git package, but I find the graph output to be a bit
too lengthy for my taste. Added `git_graph_options` setting that
can be customized by users. I prefer a more concise format:

```
"git_graph_options": "--pretty=format:'%h %ad | %s%d [%an]' --date=short"
```

Users will need to customize the syntax file based on their preferred
graph options.