Add ShellExec plugin

This plugin enables Sublime users to define shell commands in the setting file and make it available as a sublime command. 

![Sublime command menu](http://i.imgur.com/hKhp1m1.png =200x)

## What so different about this package?
What makes this package different to other shell plugins is the event hook feature that allows user to auto execute a predefined command on sublime event. E.g. After file save. 

This is especially useful for working with tests and projects that requires auto builds.

More details in the [repo](https://github.com/creativej/SublimeShellExec)