run dev server for html5 target

This PR is related to https://github.com/openfl/openfl-tools/issues/29

It's more of a experiment if nothing else. I only tested it on OSX and its working.

Add this to a project.xml:
```xml
<server host="localhost" port="8000" />
```

Then with:
```shell
openfl test html5
```

A neko server *should* start on **localhost:8000**, an the browser *should* open to that. Without defining a server, the index.html file should open in the browser as before.

---

This experiment gave me quite a bit of ideas, like using this for any target, but I just went with what was quick and dirty. I didn't want to get into haxe/neko threading, force a web framework dependency, or assume another non-haxe/neko binary was available on the system. I may already be assuming too much about nekotools.