Add a chapter on custom formatting to the stagin area

[vojtechkral]

Using cities like in json :-)
I have no idea where this could go in the structure since traits are a bit too advanced a topic for the 2nd chapter...

[japaric]

TIL: How to implement the Show trait

[japaric]

We need to introduce what "formatting argument types" are. I have opened #130 for this.

[japaric]

Thanks for the PR!

I like the example. I was wondering if we should also implement another formatting trait (like {:d}) for completeness purposes.

Order-wise, this should go after the "traits" chapter

[vojtechkral]

Ok, I'm gonna fix the issues...

I was wondering if we should also implement another formatting trait (like {:d}) for completeness purposes.

Yeah, I was thinking about that too, but the Cities example is not really suitable for that IMO. How about this:

enum StoneAgeNumber {
    One,
    Two,
    Many,
}

and have it print "one", "two", "many" for {:s} and "1", "2", ">2" for {:d}

[japaric]

@vojtechkral Sounds good to me!

[vojtechkral]

@japaric I'm in the process of updating this PR, there's a bit of a strange thing going on with the fmt::String trait, I posted about on reddit: http://redd.it/29tums

[vojtechkral]

@japaric I'm still struggling to lay out this chapter so that it makes sense. Currently only one playpen editor per page is allowed, so I'd have to move the StoneAgeNumber example into subchapter. Or perhaps I could replace the earlier code ... Do you think that the StoneAgeNumber example makes the Cities example redundant?

[japaric]

I'll re-review this later today, but I want to drop some notes now (or I might forget):

• I think the most used formatting trait is the Show trait, so we should focus on that one.
• The other traits are mostly implemented by numeric types, which allow different representations. These traits should be covered in a second example. I think an Inch(T) type may work, it'll get formatted as 1000 inches/1e3 inches or something like that.
• I've been implemented the Show trait lately, and I have noticed that if I use impl Show for Foo ... write!(f, "Foo") then using println!("{:>8}", Foo) will ignore the padding parameter! So instead of using the write! macro, one should use the f.pad("foo") method instead - as this method uses the padding information. I think this piece of information should be covered.