Add: opts.booleanBoth

[EstebanMacanek]

Add option booleanBoth (couldn't find a better name).

opts.booleanBoth - A boolean, string, or array of strings to always treat as booleans. If true will treat all double hyphenated and single hyphenated arguments without equal signs as boolean (e.g. affects --foo and -f, not --foo=bar or -f=bar). A boolean option will consume the following argument if it is the string true or false (e.g. --foo false).

It may be better to add opts.booleanSingle (e.g. affects -f, not -f=b, --foo or --foo=bar) as complementary to opts.boolean instead of opts.booleanBoth which intersects. But the idea came to me once I had already programmed everything.

I'm an amateur programmer, so the code needs improvement.

#75

[ljharb]

The issue would be the place to discuss whether this is a feature that's desired at all. It's still not clear why this is beneficial.