TODO

-   Make output<basic_ostream> check for errors, and cause outf()
    to return a negative value in that case, like printf() does.
    I prefer exceptions for that kind of thing (which is how errors
    in output to a std::string will get reported, most likely
    std::bad_alloc), but they're not turned on by default, and
    I shouldn't limit others who want to do inline error checking
    rather than use exceptions.

-   Add more tests:
    strings
    floating point
    position specifiers (e.g. %1$*2$.*3$d)
    lots of arguments (>20)
    lots of types (>20) - may not be able to make that work

-   Finish support for user-defined types, including finishing
    operator<<(ostream&, format_spec&).