feature/expanded-autonaming

This pull request contains patches to allow Scallop to supply intelligent default names for all option types:
 * Guess the names of toggles using the same method as is currently used for opts
 * Guess the names of trailArgs using the same method as is currently used for opts
 * Use a default name of "D" for props
 * Use a default name of "Props" for propsLongs

These changes should slightly reduce the number of keystrokes needed in order to get started with Scallop, as it mostly eliminates the need to type the names for your options twice.