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.