Centering formatted output.

A new flag $(B '=') has been added to the format specifier, which
allows to center the output:

```
assert(format!"|%=8d|"(1234) == "|  1234  |");
```

In case the output cannot be centered exactly it is moved slightly to
the left if $(B '-') flag is present too and to the right else:

```
assert(format!"|%=-8d|"(123) == "|  123   |");
assert(format!"|%=8d|"(123) == "|   123  |");
```
