Add nojekyll option to create .nojekyll file automatically

[shinnn]

How about nojekyll option?

When options.nojekyll is true, the gh-pages task will do:

1. Check whether the src pattern includes one or more files or directories whose name start 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 if it is unnecessary.

This PR adds the option described above and test for it.

[tschaub]

I appreciate your work on this, but I'm inclined not to add Jekyll specific behavior (though I know this plugin is named gh-pages and Jekyll is an integral part of GitHub's gh-pages handling). My justification is that I think it should be easy enough for users to create a .nojekyll file themselves (with a separate task). This plugin does quite a bit already, and I'd like to try to limit its scope.

[shinnn]

I see.
Thanks you for reviewing my PR!