Escape list markers at beginning of paragraph:

```
% pandoc -t markdown
\* ok

\+ ok

\- ok

1\. ok

a\. ok
^D
\* ok

\+ ok

\- ok

1\. ok

a\. ok
```

Here we don't need to escape because there's no space:

```
% pandoc -t markdown
\+ok

\-ok

1.ok
^D
+ok

-ok

1.ok
```

Also escape things that might become line blocks or tables:

```
% pandoc -t markdown
\| hi \|
^D
\| hi \|
```

