feat: custom separators for variadic flags

[molisani]

Resolves #74

Describe your changes
Allow variadic option for flags to be a string. If set, Stricli will use that string as a separator and call .split(separator) on every raw input (before it is parsed/validated).

For example, variadic: "," will split the raw input --flag 1,2,3 into ["1", "2", "3"] which will each be parsed separately (assuming this is a parsed number flag). This can still be combined with multiple instances of the same flag, so --flag 1,2 --flag 3 will be ["1", "2", "3"] as well.

Testing performed
Added several new test cases to achieve complete coverage.

Additional context
I've also added some additional validation to the flag specification, as the separator cannot be the empty string and should not include any whitespace.

Note: I don't know what would be the best way to indicate this behavior in the help text. I experimented with --flag <placeholder><separator><placeholder> but that got too verbose too fast.

[mkubilayk]

Note: I don't know what would be the best way to indicate this behavior in the help text. I experimented with --flag <placeholder><separator><placeholder> but that got too verbose too fast.

We could use the right aligned [...] section to mention the separator in the help text.

  -m [--mode]...           Brief                            [separator = ,]

[molisani]

Note: I don't know what would be the best way to indicate this behavior in the help text. I experimented with --flag <placeholder><separator><placeholder> but that got too verbose too fast.

We could use the right aligned [...] section to mention the separator in the help text.

  -m [--mode]...           Brief                            [separator = ,]

That works very nicely as variadic flags can't have default values, so they won't have to stack.