Add `nojekyll` option to create .nojekyll file automatically

How about `nojekyll` option?

If `options.nojekyll` is set to `true`, the `gh-pages` task will do:

1. Check whether the `src` pattern includes one or more filepath which starts with `_`.
2. If so, create `.nojekyll` file at the root of `options.base` dir and add `.nojekyll` to the `src` pattern.
3. If not, remove the existing `.nojekyll` file at the root of `options.base` dir.

This option prevents us from forgetting [to bypass Jekyll processing of Github Pages](https://github.com/blog/572-bypassing-jekyll-on-github-pages) if necessary.

This PR adds the `nojekyll` option and the test script for it.