Update defaults to match Github conventions (closes mixmark-io#424)

Refs: mixmark-io#424

README.md

@@ -50,15 +50,16 @@ var turndownService = new TurndownService({ option: 'value' })
 
 | Option                | Valid values  | Default |
 | :-------------------- | :------------ | :------ |
-| `headingStyle`        | `setext` or `atx` | `setext`  |
-| `hr`                  | Any [Thematic break](http://spec.commonmark.org/0.27/#thematic-breaks) | `* * *` |
-| `bulletListMarker`    | `-`, `+`, or `*` | `*` |
-| `codeBlockStyle`      | `indented` or `fenced` | `indented` |
+| `headingStyle`        | `setext` or `atx` | `atx` |
+| `hr`                  | Any [Thematic break](http://spec.commonmark.org/0.27/#thematic-breaks) | `---` |
+| `bulletListMarker`    | `-`, `+`, or `*` | `-` |
+| `codeBlockStyle`      | `indented` or `fenced` | `fenced` |
 | `fence`               | ` ``` ` or `~~~` | ` ``` ` |
-| `emDelimiter`         | `_` or `*` | `_` |
+| `emDelimiter`         | `_` or `*` | `*` |
 | `strongDelimiter`     | `**` or `__` | `**` |
 | `linkStyle`           | `inlined` or `referenced` | `inlined` |
 | `linkReferenceStyle`  | `full`, `collapsed`, or `shortcut` | `full` |
+| `br`                  | `  ` or `\n` | `\n` |
 | `preformattedCode`    | `false` or [`true`](https://github.com/lucthev/collapse-whitespace/issues/16) | `false` |
 
 ### Advanced Options

src/turndown.js

@@ -25,16 +25,16 @@ export default function TurndownService (options) {
 
   var defaults = {
     rules: COMMONMARK_RULES,
-    headingStyle: 'setext',
-    hr: '* * *',
-    bulletListMarker: '*',
-    codeBlockStyle: 'indented',
+    headingStyle: 'atx',
+    hr: '---',
+    bulletListMarker: '-',
+    codeBlockStyle: 'fence',
     fence: '```',
-    emDelimiter: '_',
+    emDelimiter: '*',
     strongDelimiter: '**',
     linkStyle: 'inlined',
     linkReferenceStyle: 'full',
-    br: '  ',
+    br: '\n',
     preformattedCode: false,
     blankReplacement: function (content, node) {
       return node.isBlock ? '\n\n' : ''