Add provision to apply functions to variables within snippets

[thekalinga]

• VSCode Version:

Version 1.9.0-insider
Commit 32222e2
Date 2016-12-16T07:18:07.856Z
Shell 1.4.6
Renderer 53.0.2785.143
Node 6.5.0

• OS Version:

Ubuntu 14.04

Steps to Reproduce:

Currently snippets does not have any way to apply functions to the variable being edited.

For eg., if the user selects ng2-component template using this plugin, user needs to enter the same name in multiple places

1. in lowercase for the selector name
2. in Camelcase for the class name

If vscode supports applying functions to the template variables names, the extension developer can write functions to update the rest of the template using this the text captured within this variable

[unional]

Besides function, I always want to use date string for today.
Should it be a separate request?

[thekalinga]

If you have ability to write code, its trivial to get date.

This is the holy grail.

[jrieken]

closing this as dupe of #6920