increase child process buffer size

When running `htmllint` on 100+ HTML files with lots of errors, I received a `stderr maxBuffer exceeded` error:
```
[11:27 PM]-[9577]-[justin@justin]-[~/projects/poses] (html-validation-fixes @ 058a7b9 ⚡ )
$ grunt htmllint
Running "htmllint:all" (htmllint) task
>> Error: stderr maxBuffer exceeded.
Warning: Task "htmllint:all" failed. Use --force to continue.

Aborted due to warnings.
```
Bummer!

According to [the child_process docs](http://nodejs.org/api/child_process.html#child_process_child_process_exec_command_options_callback),  `exec()` accepts an options hash that allows you to specify the `maxBuffer`. Increasing it 100x (20MB vs. the default 200KB) seemed to do the trick for me.

The only downside to this I can imagine is more memory usage. :shrugs: …worksforme!