_mocha: Allow boolean --reporter-options #1754

Previously "mocha --reporter-options foo" would blow up because an equal sign is required. This is quite unhandy for reporters that need to accept boolean (flag-ish) options. This commit changes that so that a value-less reporter option will be interpreted as having a value of true. This means that --reporter-options foo=bar,quux will turn into { foo: 'bar', quux: true }