Add support for variadic options

[shadowspawn]

Pull Request

Problem

People would like to be able to use variadic options, an option which can take more than one value from the following arguments.

See #571

Solution

1. Syntax:

program
   .option('-m, --mods <mod...>'),
   .option('-o, --optional [opts...]')

Being cautious, this makes the change a breaking change in case people have put dots after a comma separated list for example.

2. If a value or values are specified then the option value is an array.

3. The first option argument is parsed in the same way as if the ... was not specified.

4. Subsequent option arguments are processed as for optional options ([value]), so a leading - or -- indicates an option and not a further argument. A lone -- stops the option processing as normal.

5. The variadic behaviour triggers for stand-alone options with the first value in the next argument, and not for options where the option value is included in the argument.

These do not trigger variadic processing:

$ program -mvalue operand
$ program --optional=value operand

6. Repeated use of the option will accumulate the values.

These are equivalent:

$ program --mods first second
$ program --mods first --mods second

Example:

$ transpile --include *.js --exclude *.test.js -- operand

ChangeLog

• add support for variadic options (Add support for variadic options #1250)

[shadowspawn]

Using variadic options will cause some complications for caller to clarify their intent under some circumstances, in similar ways to options with optional values. So I don't want to promote it too much, it should only be used when compelling benefits.

I am thinking it will be mentioned briefly in the README with a longer explanation in a separate document.

[shadowspawn]

Added README and ready for review.

Fixed a few spelling mistakes as well, so some extra files touched.

[abetomo]

LGTM!