Attribute selectors probably should use quotes around the attribute value

[nateweiss]

Hi,

First, thanks for this library. I noticed that if you're using the optional 'attribute' selector style, the resulting selector is not quoted, which can lead to some errors if it decides to write a selector such as a[href=some-page.htm] or similar.

I've added a modification locally that just adds quotes (so a[href="some-page.htm"] or whatever), would you like a Pull Request with that change?

Related, I've also added an attribute_blacklist (defaults to ['id', 'class']) and attribute_whitelist option (defaults to []) option, at the same level as the existing selectors option. Basically, my use case wants to "prefer" selectors that are based on link targets, so now my usage is like:

new CssSelectorGenerator
  selectors: ['id', 'class', 'attribute', 'tag', 'nthchild']
  attribute_whitelist: ['href','src']

I've also added a class_blacklist option (defaults to []), if there are CSS class names that shouldn't be considered by getClassSelectors. In my case there are a handful of "internal" class names that should be ignored when generating a selector.

Let me know if you'd like PRs for any of these changes. They are all pretty trivial in terms of the actual code changes.

[fczbkk]

Thanks, I have merged the changes.