Add support for more selectors

[antiagainst]

1. Add support for getting CSS attribute value.

This commit add a new key 'attr' in dashing.json selector objects.
If "attr": "attribute" exists, dashing will get the value of the
"attribute" of the current selected node and use it instead of the
node text as the base name for transforms.

E.g.,

<a id="wow">link</a> together with "attr": "id" will get "wow"
instead of "link".

2. Add support for requiring text content when matching.

With this commit, we can further require the node selected with
CSS selector to match certain text regexp.

E.g.,

<h2>This is a title</h2> with "requiretext": "^This is" will
considered as a match, while with "requiretext": "^No", it is not.

3. Add support for multiple transforms of the same CSS selector.
   With this commit, we can supply multiple transforms for the same
   CSS selector. It enables treating the same CSS selector with
   different texts separately.

E.g.,

<h2>Chapter X</h2>
<h2>Section Y</h2>

with

"h2": [
  {
    "requiretext": "^Chapter",
    "Type": "Module"
  },
  {
    "requiretext": "^Section",
    "Type": "Section"
  }
]

Will treat "X" as Module and "Y" as "Section".

[antiagainst]

These changes are used to generate the Vulkan docset: Kapeli/Dash-User-Contributions#1238.

The dashing.json is: https://github.com/antiagainst/dashing/blob/vulkan/examples/vulkan/dashing.json

[technosophos]

Thank you! I'm just not catching up on a backlog of PRs from across my projects. I appreciate your patch and am sorry it took so long.