New skip function

Skip function allow break current test and report some message.
But this test count as passed.
```lua
function test_optional()
  if not has_module_foo then skip"Module foo not found" end
  -- do test
end
```
See: https://travis-ci.org/moteus/lua-pop3 (build #16.2)
Test could not be passed because of lpeg. So i skip them.

Also this commit has few minor fixes.
lunitx ignores errors in tests
lunit.sh -t ... rises error

I think there still error in lunit.sh.
I test it on MinGW.
When i run `lunit.sh -t * test.lua` $options contain `-t test.lua test.lua`