Accept an Array (as well as string) for options.exec

When an array is given, don't do any splitting on spaces.

This is a simpler alternative to #418 which doesn't affect the CLI interface, just the API. Although it does fix a bug in the CLI's `{{filename}}` and `{{pwd}}` expansion as a side effect, which I've added a test for too.
Great, thanks for all your hard work :)

As for docs, yeah it might be worth mentioning. I just had a look from the frontpage and the most appropriate place I found via that is https://github.com/remy/nodemon/blob/master/doc/cli/config.txt

Perhaps something like:

> The `exec` option allows multiple arguments - it splits the string on space characters. If you're using the .json config, you can pass an array of strings instead of a single string, in which case nodemon won't do any further processing.