Support shorter flag syntax

[brandonkal]

Currently adding a flag is quite verbose:

## purchase (price)

> Calculate the total price of something.

**OPTIONS**
* tax
    * flags: -t --tax
    * type: number
    * desc: What's the tax?

~~~sh
TAX=${tax:-1} # Fallback to 1 if not supplied
echo "Total: $(($price * $TAX))"
~~~

We end needing to write out the flag name twice and use 4 lines.
I'd like to propose support for a shorter readable syntax:

## purchase (price)

> Calculate the total price of something.

**OPTIONS**
* flags: -t --tax |number| What's the tax?

~~~sh
TAX=${tax:-1} # Fallback to 1 if not supplied
echo "Total: $(($price * $TAX))"
~~~

If no type is specified, the |number| part can be omitted.

[brandonkal]

This also has the benefit of looking closer to the output of the CLI help where flag and description are printed on the same line.

[MggMuggins]

Might I suggest flag instead of flags, since each line defines a flag and it will be repeated many times? Ex:

## purchase (price)

> Calculate the total price of something.

**OPTIONS**
* flag: -t --tax |number| What's the tax?
* flag: ...

~~~sh
TAX=${tax:-1} # Fallback to 1 if not supplied
echo "Total: $(($price * $TAX))"
~~~

[slavaGanzin]

And default value as well can go into square braces like:

flag: -t --tax |number| [-1] What's the tax?

[dswd]

+1 for shorter options strings

How about the DocOpt syntax? There is even a Rust implementation that you could take code from.