Dynamically adjust width of long option name column

[remkop]

The current layout (up to and including 3.0-alpha-6) has a fixed width long option name column:
this column is always 24 characters, even if none of the options have a long option name.

This gives weird-looking usage help messages in some cases. For example:

def cli = new CliBuilder(name:'ls')
cli.a('display all files')
cli.l('use a long listing format')
cli.t('sort by modification time')
def options = cli.parse(args)
...

The usage message for this example is:

Usage: ls [-alt]
  -a                          display all files
  -l                          use a long listing format
  -t                          sort by modification time

Picocli should adjust the width of the long option name column to the longest name (max 24).

Side note: the effective max width for long options is 20, since 4 characters out of that 24 are padding: 1 initial leading space, and 3 trailing spaces. Long options that are longer than 20 characters cause the long option column to "overflow" into the description column, and description to be displayed on the next line.