Fix escaping with surrounds

E.g.:
```
$ echo '*\*test*' | smu
```

Before:
```
<p>
<em>\</em>test*</p>
```

Now:
```
<p>
<em>*test</em></p>
```

I also added an example to testdoc motivating not enabling "process" for backticks, since I initially thought that might be the fix. Include or discard it as you please.