Support aliases

[MisanthropicBit]

Fixes #158.

• fdescribe => describe.only
• xdescribe => describe.skip
• it.eachtable(name, fn)
• it.failing(name, fn, timeout) and test.failing(name, fn, timeout)
• it.failing.each(name, fn, timeout) and test.failing.each(name, fn, timeout)
• it.todo(name) and test.todo(name)
• test.concurrent.each(table)(name, fn, timeout)
• test.concurrent.only.each(table)(name, fn)
• test.concurrent.skip.each(table)(name, fn)
• test.failing.each(name, fn, timeout)
• test.only.failing(name, fn, timeout)
• test.skip.failing(name, fn, timeout)
• test.skip.each(table)(name, fn)