Add exports of one, all helpers

These helper functions are useful when writing custom handlers.

Closes GH-54.

Reviewed-by: Titus Wormer <[email protected]>

Author: ignatiusreza

index.js

@@ -1 +1,3 @@
+export {all} from './lib/all.js'
+export {one} from './lib/one.js'
 export {toHast} from './lib/index.js'

readme.md

@@ -54,7 +54,7 @@ Which when running with `node example` yields:
 
 ## API
 
-This package exports the following identifiers: `toHast`.
+This package exports the following identifiers: `toHast`, `all`, `one`
 There is no default export.
 
 ### `toHast(node[, options])`
@@ -217,6 +217,18 @@ Yields, in [hast][] (**note**: the `pre` and `language-js` class are normal
 }
 ```
 
+### `all(h, parent)`
+
+Helper function for writing custom handlers passed to `options.handlers`.
+Pass it `h` and a parent node (mdast) and it will turn the node’s children into
+an array of transformed nodes (hast).
+
+### `one(h, node, parent)`
+
+Helper function for writing custom handlers passed to `options.handlers`.
+Pass it `h`, a `node`, and its `parent` (mdast) and it will turn `node` into
+hast content.
+
 ## Security
 
 Use of `mdast-util-to-hast` can open you up to a