Reporter HTML: listen to new 'EVENT_TEST_SKIPPED` event

[juergba]

Description

Currently a failing hook changes the shape of a test run and related tests just disappear. They are neither listed in the reporter output nor shown in the test summary.
#4221 / #4233 add new test events: EVENT_TEST_SKIPPED.

ToDo in reporter HTML:

• listen to EVENT_TEST_SKIPPED
• report skipped tests in color red (similar to pending tests)
• mocha-stats:
  • add pending
  • add skipped
  • add total (stats.tests)
  • fix passesLink: should only show passing tests, without pending tests
  • fix progress
• remove "<=IE7" stuff