Raise UndefinedPromptMode when --prompt has no value

[taekop]

Fixes #910

irb --prompt (also --prompt-mode and --prompt=) with no mode crashed in parse_opts with undefined method 'upcase' for nil (NoMethodError). It now raises IRB::UndefinedPromptMode, the same error IRB already uses for an unknown mode such as irb --prompt bogus.

Listing the available modes is left out: modes can still be added by .irbrc after option parsing, so the list isn't final at that point.

Testing

• Added test_prompt_option to test/irb/test_init.rb. It covers --prompt simple, --prompt-mode=inf-ruby and the missing-value case. On master it fails with the NoMethodError, and with this change it passes.
• bundle exec rake test: 411 tests, 0 failures, 0 errors (3 omissions).
• bundle exec rubocop lib/irb/init.rb test/irb/test_init.rb: no offenses.
• Manual check: irb --prompt now prints Undefined prompt mode(). (IRB::UndefinedPromptMode). --prompt simple, --prompt=classic, --prompt-mode inf-ruby and --noprompt work as before.