Make custom tags easier

[eemeli]

This is another continuation of #77, #79, and #80, and replaces some of the solutions previously implemented. Specifically, this PR is doing three things:

1. Deprecate the class attribute of tags, replacing it with the more generic identify(value).
2. Turn the string stringifier into a generic scalar stringifier, fallback to it by default, and export it as stringify() from yaml/schema.
3. Improve the documentation on writing custom tags.

In total, this means that e.g. RegExp and shared Symbol tags could now be implemented as:

import { stringify } from 'yaml/schema'

const regexp = {
  identify: value => value instanceof RegExp,
  tag: '!re',
  resolve(doc, cst) {
    const match = cst.strValue.match(/^\/([\s\S]+)\/([gimuy]*)$/)
    return new RegExp(match[1], match[2])
  }
}

const sharedSymbol = {
  identify: value => value.constructor === Symbol,
  tag: '!symbol/shared',
  resolve: (doc, cst) => Symbol.for(cst.strValue),
  stringify(item, ctx, onComment, onChompKeep) {
    const key = Symbol.keyFor(item.value)
    if (key === undefined)
      throw new Error('Only shared symbols are supported')
    return stringify({ value: key }, ctx, onComment, onChompKeep)
  }
}

[eemeli]

Rebased on top of master.

@isaacs Note that this PR removes the obj.constructor === t.class check that #79 introduced.

[isaacs]

I think this is a better API for custom tags.