Add Nicholas Nethercote to .mailmap.

Before:
```
$ git shortlog -se | grep Nethercote
   517	Nicholas Nethercote <n.nethercote@gmail.com>
     2	Nicholas Nethercote <nnethercote@apple.com>
   560	Nicholas Nethercote <nnethercote@mozilla.com>
```
After
```
$ git shortlog -se | grep Nethercote
  1079	Nicholas Nethercote <n.nethercote@gmail.com>
```

Author: nnethercote

.mailmap

@@ -396,6 +396,8 @@ Nathaniel Herman <nherman@post.harvard.edu> Nathaniel Herman <nherman@college.ha
 Neil Pankey <npankey@gmail.com> <neil@wire.im>
 Ngo Iok Ui (Wu Yu Wei) <wusyong9104@gmail.com>
 Nicholas Baron <nicholas.baron.ten@gmail.com>
+Nicholas Nethercote <n.nethercote@gmail.com> <nnethercote@apple.com>
+Nicholas Nethercote <n.nethercote@gmail.com> <nnethercote@mozilla.com>
 Nick Platt <platt.nicholas@gmail.com>
 Niclas Schwarzlose <15schnic@gmail.com>
 Nicolas Abram <abramlujan@gmail.com>