improve example help text. support example searches that include adafruit_

[FoamyGuy]

Adding some improvements suggested in #251 and on discord.

circup example --list now works when the user includes the full library name with "adafruit_" in it.

$ circup example adafruit_mlx --list
mlx90393/mlx90393_simpletest
mlx90393/mlx90393_temperature
mlx90395/mlx90395_simpletest
mlx90395/mlx90395_tuning
mlx90614/mlx90614_simpletest
mlx90632/mlx90632_simpletest
mlx90640/mlx90640_camtest
mlx90640/mlx90640_pil
mlx90640/mlx90640_pygamer
mlx90640/mlx90640_simpletest

Also added more details to example --help output with examples of different usages of the command.

Mentioned in the issue, but haven't looked into yet, still planning to: circup example --list not showing results if bundle has not been downloaded.

[tannewt]

Makes sense to me!