Make the colon optional when using Python-style format specifiers

[amarz45]

Essentially, I am asking for these three printfln lines of code to do the same thing:

mem_unit := 'K'
fmt.printfln("%ciB", mem_unit) // this is valid
fmt.printfln("{:c}iB", mem_unit) // this is valid
fmt.printfln("{c}iB", mem_unit) // this is invalid

I think the compiler should treat a format specifier with braces and without a colon the same as if there was a colon after the opening brace.