Refactor option value processing to improve maintainability and fix bugs

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There is some subtle code in addOption which is hard to make sense of, and has some bugs. If is solid on options used once on command line, but from code inspection I was able to reproduce some logic errors affecting repeated options for boolean with a non-boolean default value (bit obscure!) and optional options.

The problems have been present for a long time. I was able to reproduce with v2.20.0 and traced code back to at least v1.1.0. The issues have not been generating reports, and the motivation for changing the code is as much to make the code easier to work on as to improve the behaviour.

commander.js/lib/command.js

Line 515 in 43f4743

addOption(option) {

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Example bug 1

program.option('-f, --flag', 'boolean option with a non-boolean default', 'flag seen')
program.parse(process.argv);
console.log(program.opts());

% node bug1.js -f
{ flag: 'flag seen' }
defaults % node bug1.js -f -f
{ flag: undefined }

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Example bug 2

program.option('-o, --optional [value]')
program.parse(process.argv);
console.log(program.opts());

defaults % node bug2 -o
{ optional: true }
defaults % node bug2 -o -o foo
{ optional: 'foo' }
defaults % node bug2 -o -o foo -o
{ optional: 'foo' }

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Commander v9 has been released.