allow value delimiter to be changed

[kbknapp]

Relates to #348 and #352 and requires those to be implemented first.

This would default to , (comma), but allow the user to change the value delimiter to any other arbitrary character using something like Arg::value_delim(char)

For example, if I wanted to change the delimiter to ; (semicolon) I could use Arg::value_delim(';') and then val1;val2;val3 would be parsed as 3 values but val1,val2,val3 would only be parsed as a single value.

[Vinatorul]

Wow, this is very cool! Great idea, I will do at free time closer to end of the month, if it willn't be done.

I will be able to change my custom validation in minesweeper.

[kbknapp]

Requires #348

[kbknapp]

Closed with #388