Add support for Chrome _initiator block

[dnnrly]

Is your feature request related to a problem? Please describe.
Chrome has additional _initiator block. This seems to include information about what has invoked each network fall.

Describe the solution you'd like
It would be great if we could see which scripts or user events initiate network calls in our diagrams

Describe alternatives you've considered
Nothing yet. This appears to be specific to Chrome but this information might be present in other browsers

Additional context
n/a

[DBX12]

HI @dnnrly,
hoofli sounds like an interesting project and I would like to help out here. I'll hand in a pull request shortly. Do you have an idea how you want to show the _initator on the diagram? I think the most interesting part is the type property of the _initiator object since it shows who started the request. The other parameters are probably interesting as well, but will clutter the diagram. Maybe they can be shown when a certain flag is provided to hoofli?
For the type, I think I would go with different colors for the different types. I could identify the values script, parser and other so far.

[dnnrly]

I don't really have a strong opinion as long as it makes it clear what's going on. Why don't you have a try and if you're not sure then propose some options.

[DBX12]

I think the color-coding does a good job and showing what caused a request. I've put it up as PR so you can have a look :)