Remove and prevent DOM access in block attributes matchers

[aduth]

Closes #608

This pull request seeks to remove existing attributes matchers which operate on the DOM; currently image and quote blocks pulling attribute values from element class names. Instead, these attributes are to be encoded into the block comment. Further, it seeks to prevent future attributes created in this pattern by applying an internal flag to acceptable attribute matcher creators (attr, children, etc).

Copying an included code comment which elaborates:

// Matchers are implemented as functions that receive a DOM node from
// which to select data. Use of the DOM is incidental and we shouldn't
// guarantee a contract that this be provided, else block implementers
// may feel compelled to use the node. Instead, matchers are intended
// as a generic interface to query data from any tree shape. Here we
// pick only matchers which include an internal flag.

Testing instructions:

Ensure that tests pass:

npm test

Verify that image alignment and quote styles are still respected, including after changing the values and switching back and forth between Visual and HTML modes.

[aduth]

Ideally we'd not need to coerce style to a number, but instead incorporate into attributes (#609) or serialize comment attributes as typed-ish JSON (#482 (comment)).

[mtias]

I like this.

[aduth]

Rebased with consideration of #616, #623, and #626.

Will plan to merge shortly.