Add support for registering custom merge (mustache) functions

Currently, Skuid provides support in merge syntax for things like $Model, $Label, etc. as well as some stock functions like en/decodeUrl.  

Would like to see support added for registering custom merge/mustache properties and functions.

Examples:
1) register $MyAppInfo which is an in-memory object
Usage: {{$MyAppInfo.myProperty}}
2) register currentTime which would be a function to automatically merge in the current time
Usage: {{#currentTime}}The time is: {{/currentTime}}
3) register doSomethingCool which would be a function to automatically merge in something cool
Usage: {{#doSomethingCool}}{{MyField__c}}{{/doSomethingCool}}

Thank you!