Fix #205: Handle optparse errors gracefully

[avdv]

Description

Don't bail out with a stacktrace for invalid options.

before:

ls -2
Traceback (most recent call last):
	5: from /home/claudio/.gem/ruby/2.5.0/bin/colorls:23:in `<main>'
	4: from /home/claudio/.gem/ruby/2.5.0/bin/colorls:23:in `load'
	3: from /home/claudio/.gem/ruby/2.5.0/gems/colorls-1.1.2/exe/colorls:5:in `<top (required)>'
	2: from /home/claudio/.gem/ruby/2.5.0/gems/colorls-1.1.2/exe/colorls:5:in `new'
	1: from /home/claudio/.gem/ruby/2.5.0/gems/colorls-1.1.2/lib/colorls/flags.rb:24:in `initialize'
/home/claudio/.gem/ruby/2.5.0/gems/colorls-1.1.2/lib/colorls/flags.rb:161:in `parse_options': invalid option: -2 (OptionParser::InvalidOption)

after:

$ colorls -2
colorls: invalid option: -2
See 'colorls --help'.

• Relevant Issues : Nicer output when passing invalid options #205
• Relevant PRs : (none)
• Type of change :
  • New feature
  • Bug fix for existing feature
  • Code quality improvement
  • Addition or Improvement of tests
  • Addition or Improvement of documentation