proposal: flag: Introduce functions for defining rune flag

Currently the flag package has functions for parsing string.

In order use a rune flag one has to

1. define it as string flag
2. check rune count using utf8.RuneCountInString to know if is has one or more rune.
3. if rune count is more than one, produce an error or call flag.Usage and exit. ( But the usage information
   that flag.Usage gives will contain -flag string for that flag. so it requires to add `rune` to that flag's usage
   information to get flag.Usage show -flag rune for that flag's usage information. )
4. type cast it to []rune
5. get the first rune by using index 0

[ianlancetaylor]

The need for a flag that accepts a single rune seems 1) rare; 2) easy to implement by implementing the flag.Value interface. What is the argument for putting this in the standard library? See https://golang.org/doc/faq#x_in_std.

[rsc]

I'm at a loss to remember any program I've ever used that took a flag argument that was itself (the argument) required to be a single letter.

Note that you can implement this yourself using the new flag.Func (will be in Go 1.16) or using flag.Value.

[rsc]

(Retracted by author.)