Export slugify function (#566)

* Prefix slug with dash

* Update test snapshots

* Add changeset

* Export slugify function

* Revert test changes

Author: jackyef

.changeset/tall-crabs-flow.md

@@ -0,0 +1,21 @@
+---
+"markdown-to-jsx": patch
+---
+
+Browsers assign element with `id` to the global scope using the value as the variable name. E.g.: `<h1 id="analytics">` can be referenced via `window.analytics`.
+This can be a problem when a name conflict happens. For instance, pages that expect `analytics.push()` to be a function will stop working if the an element with an `id` of `analytics` exists in the page.
+
+In this change, we export the `slugify` function so that users can easily augment it. 
+This can be used to avoid variable name conflicts by giving the element a different `id`.
+
+```js
+import { slugify } from 'markdown-to-jsx';
+
+options={{
+  slugify: str => {
+    let result = slugify(str)
+
+    return result ? '-' + str : result;
+  }
+}}
+```

index.tsx

@@ -588,7 +588,7 @@ function unquote(str: string) {
 
 // based on https://stackoverflow.com/a/18123682/1141611
 // not complete, but probably good enough
-function slugify(str: string) {
+export function slugify(str: string) {
   return str
     .replace(/[ÀÁÂÃÄÅàáâãäåæÆ]/g, 'a')
     .replace(/[çÇ]/g, 'c')