add EnumSliceVar flag

[dogancanbakir]

Closes #127.

Note: I didn't want to introduce any changes that would break the EnumVar API. We can merge those changes in the future.

[tarunKoyalwar]

lgtm !

$ ./basic -h severity


Usage:
  ./basic [flags]

Flags:
   -s, -severity value  severity of the scan (default high)

$ ./basic -s high,low
Got Severity: [high low]

[Mzack9999]

lgtm!

I'm thinking if it makes sense to make the split/join placeholder configurable for example via callbacks, for example:

type SplitFunc func(s string) []string
type JoinFunc func(s []string) string