Rework options description to clarify option types and custom processing

[shadowspawn]

Mainly rewriting options coverage in README:

• rewrite description of function for custom processing (Triage: Coercion vs Accumulation #943), previously called "Coercion"
• focus on common option types, boolean and option taking <value>
• cover option with optional [value] separately under "other" types
• mention short option is single character (Throw an error when the short flag for an option for longer than one character #937)
• explicitly cover defaults (Provide info about default values on the main github page #884)
• added explanation of declaring program using new Command (Document Command class in readme #790)
• added some extra info about boolean options (Defining boolean options via Command#option() defaults to undefined, not false. #574)

Still to come: small changes to version option description

Comments welcome from anyone.

[shadowspawn]

I have added about 100 lines to length of README, but hopefully clearer!

(I didn't refactor the regular expression support in with the rest of the options. Future work.)

(I think we should add a table of contents, but after tidying more of the README so the headings make better sense. Future work.)

[ozbillwang]

Looks much better. 👍

[shadowspawn]

I had a look at "Insights > Traffic" and am surprised how much traffic examples/ gets. In last 2 weeks the README got 2912 and examples/ got 518 unique visitors.

Since the examples are being used, I think I'll add some more info to the new example files I added like "as shown in README", and add the expected output in a comment. However, this PR can be reviewed or merged before I do that, as examples can be added separately.

[ozbillwang]

One suggestion.

.option('-c, --cheese <type>', 'add the specified type of cheese', 'blue');

Until now, I still don't fully understand how this option <type> to be used.

I accidentally wrote a code as below (without <type>)

 .option('-c, --cheese', 'add the specified type of cheese', 'blue');

Then this option doesn't work as expected.

But I am not sure what should be put, currently just copy it as <type>. There is no any sample to change this part in README

[shadowspawn]

Thanks @ozbillwang. I have in the early description:

The two most used option types are a boolean flag, and an option 
which takes a value (declared using angle brackets)

but do not clarify when describing default values:

You can specify a default value for an option which takes a value.

Is it mainly the default values case you tripped over? Or do you think it would be helpful to expand on "takes a value" in both places?

[shadowspawn]

(Updated examples to include README reference and example output.)

[abetomo]

Thank you! 👍