Allow htmltidy to use a system tidy if available.

While I wasn't excited about adding in a dependency when there were no others it seemed to be the quickest and least intrusive option. It could absolutely be done by an exec call:
```
child_process.exec('command -v tidy', {}, function() { ... });
```
But that would require a bit more tweaking of the existsSync call. If you'd prefer, I can do that instead.

I ended up doing this because the supplied binary of tidy for linux didn't work for me and it took me a good while to figure out that that was the issue. Unsure if it is 32/64 bit or just different linux bundling. I figured the smartest solution was to allow the user ot install a system tidy and have the toolset default to it.