Add support for `hal+json` mime types

I know this has been raised in the past with #19 but I'm submitting it again anyway because:

1) I needed to support this
2) Pull request #19 is over a year old and will not apply to master
3) This is seems to be a popular request so hopefully this pull-request will be useful to others
4) It might help if someone wants to build on this and support configurable mime types

For what it's worth, I'd really like to see support for `application/hal+json` mime types out of the box (even if you add configurable mime types). It's becoming a popular format that I think should be supported by your tool.