Enable footnotes by default

We're already doing the work to aggregate them, so might as well just
enable them by default to reduce cognitive load. It's a specific pattern
that won't trigger accidentally.

__tests__/index.js

@@ -520,7 +520,7 @@ describe('markdown-to-jsx', () => {
                 'foo[^abc] bar',
                 '',
                 '[^abc]: Baz baz',
-            ].join('\n'), {footnotes: true}));
+            ].join('\n')));
 
             const elementNode = ReactDOM.findDOMNode(element);
 
@@ -546,7 +546,7 @@ describe('markdown-to-jsx', () => {
                 'foo[^abc] bar',
                 '',
                 '[^abc]: Baz baz',
-            ].join('\n'), {footnotes: true}));
+            ].join('\n')));
 
             const elementNode = ReactDOM.findDOMNode(element);
             const definitions = elementNode.children[1];
@@ -563,7 +563,7 @@ describe('markdown-to-jsx', () => {
                 'foo[^abc] bar',
                 '',
                 '[^abc]: Baz',
-            ].join('\n'), {footnotes: true}));
+            ].join('\n')));
 
             const elementNode = ReactDOM.findDOMNode(element);
             const definitions = elementNode.children[1];

index.js

@@ -294,6 +294,7 @@ export default function markdownToJSX(markdown, remarkOptions = {}) {
     let ast;
 
     remarkOptions.position = remarkOptions.position || false;
+    remarkOptions.footnotes = remarkOptions.footnotes || true;
 
     try {
         ast = parse(markdown, remarkOptions);