Center alignment for fmt

Use '^' to specify center alignment in format strings.

```
fmt!( "[{:^5s}]", "Hi" ) -> "[ Hi  ]"
fmt!( "[{:^5s}]", "H" )  -> "[  H  ]"
fmt!( "[{:^5d}]", 1i )   -> "[  1  ]"
fmt!( "[{:^5d}]", -1i )  -> "[ -1  ]"
fmt!( "[{:^6d}]", 1i )   -> "[  1   ]"
fmt!( "[{:^6d}]", -1i )  -> "[  -1  ]"
```

If the padding is odd then the padding on the right will be one
character longer than the padding on the left.
Stylistically we normally don't put spaces around the parens in tuples