Better time zone offset formatting.

[mbostock]

With #4, we can now parse four time zone offset formats:

• ±hhmm
• ±hh:mm
• ±hh
• Z

However, we can only format two of these (the first using %Z, and the last using a literal Z). It would be nice to format the other two as well, but I’m unsure how to represent these in the format syntax. Perhaps ±hh could be a new (lowercase) z directive, which behaves identically to Z when parsing (akin to e & d, H & I)?