Feature Request: Add ability to specify delimiter for slice flags

[pcman312]

When specifying a slice flag:

Strs []string `short:"s" long:"strs" description:"..."

there doesn't appear to be a way to specify a delimiter to use to split the provided strings into the string slice. For instance, if the above flag is specified:

./program -s one,two -s three

Results in:

0: one,two
1: three

Instead of:

0: one
1: two
2: three

The typical way to do this is to specify -s multiple times. Adding the ability to split the arguments by a delimiter would be a big usability improvement, particularly when you need to specify many arguments into that slice.

There currently is the env-delim option which appears to have a similar functionality. I propose to add a delim option that will split the flag values by the given delimiter.

[oguzyildizz]

Also, I would personally add a size tag that would enforce slice (or maybe additionally, string size) arguments. Could be a range or a single integer. e.g. size:"3" size:"1,-" size:"-,4" size"3,5"