Refactor exports

[eemeli]

Closes #94; see also #89.

After this, the public API will be:

const YAML = require('yaml')
const parseCST = require('yaml/parse-cst')

const {
  binaryOptions, boolOptions, nullOptions, strOptions,
  Pair,
  Scalar,
  Schema,
  YAMLMap,
  YAMLSeq
} = require('yaml/types')

const {
  findPair,
  parseMap, parseSeq,
  stringifyNumber, stringifyString,
  toJSON,
  Type,
  YAMLReferenceError, YAMLSemanticError, YAMLSyntaxError, YAMLWarning
} = require('yaml/util')

The previous endpoints (e.g. yaml/schema, yaml/types/omap, etc.) will still work, but they should be considered deprecated and liable to be removed in the next major update.

The built-in custom tags are now available using string identifiers, so they don't need to be publicly exported, e.g. YAML.parse(src, { tags: ['intTime', 'omap'] })

In related changes, this PR drops babel-plugin-add-module-exports, using instead a custom non-compiled main export, and improves the API for writing custom types.

Edit: Updated API description with changes made in 4f9d104