Add "Run Apps" for inovoking external tools from ST

It's a typical feature in IDEs such as Visual studio, Eclipse, ... Usually called "External Tools".

Very simple implementation: user can add customized applications, and then the commands will appear in command pallete which is very easy to invoke through "ctrl+shift+p".
Merge pull request #3877 from liuhewei/master

Add "Run Apps" for inovoking external tools from ST