Fix deploy command to find custom deployers

Add `nanoc deploy -D|--deployers` to list available deployers, instead of
building a dynamic command description.

Fixes #256
@ddfreyne said --list-deployers is nicer
Tidy up
Make --list & --list-deployers exclusive
List deployers using same format as deploy targets
Plurals, repetitions, suffixes
Merge pull request #257 from cdlm/fix-custom-deployers

Fix deploy to detect custom deployers