Allow piping commands in the exec

This is kinda awesome, because it switches to using `sh` to execute the command (windows testing to be completed), it means you can do things like this (slightly contrived examples):

```
# convert ES6 files to ES5 via 6to5, and then 
nodemon -x "6to5 src/*.js -d build/ && browserify -e build/main.js -o bundle.js | tee build.logs"

# rebuild and deploy your docker environment on file change
nodemon -x "fig build web && fig kill web && fig start web"

# make test then install install if successful on all code changes in cwd of .c and .h files
nodemon -x "make test && make install" -e c,h
```

Fixes #435