Allow a user to pass an IO object when adding a font

This PR lets 

```ruby
require 'bundler'
Bundler.setup
require 'prawn'
require 'open-uri'
pdf = Prawn::Document.new
pdf.font_families.update(
  "MyFont" => {
    format: 'ttf',
    bold: open("https://mybucket.s3.amazonaws.com/myfont.ttf")
  }
)
pdf.font("MyFont", :style => :bold)
pdf.text "Some bold text"
pdf.render_file 'output.pdf'
```

Work as expected. The big thing to notice is when passing in an IO object we need to also pass in a format: key so Prawn knows what kind of format the font is. Normally prawn uses the file extension at the end of a path to determine what format the file is in, TTF, DFONT or AFM. Really I don't think this is ideal either, where I would prefer to see the TTF class be able to tell you if a given binary stream is a TTF file or not (really asking the class if it can work with said binary stream). I glanced at the TTF spec and it didn't look incredibly easy (no simple magic number at the head of the file).

The other thing I notice is the `name` variable isn't really the font name so much as either the path the user supplied or the IO object for the font, I didn't refactor all the methods to use a more correct name but I did leave a comment in the method. If we think it's worth doing now I can try to make that update but I didn't want to risk mucking up the font code as this is the first time I have really gotten into it.

I also changed the gemfile to require a more recent commit of TTFunk as mentioned in [TTFunk Pull #17](https://github.com/prawnpdf/ttfunk/pull/17), once we are ready to merge the code we can remove that and instead cut a new release of TTFunk and make prawn require that release.

Any thoughts? It works, but it's not quite ideal.
We totally can, the reason I didn't is I wasn't sure how to handle updating [#find_font](https://github.com/prawnpdf/prawn/blob/io-fonts/lib/prawn/font.rb#L231-L241) which is where that name variable comes from, I would think we want the variable name to be the same all the way down the stack, but I'm not 100% sure I'm yet comfortable messing with code that interacts with the font_registry. It looks like we are building up our registry key using the font 'name' but really the location should be in there (to cache multiple hits to the same URL or file path I'm sure). I'll give it a shot and go from there.