User Mustache syntax for string interpolation

Deprecate previous syntax
Specify .GlobalEnv for call to whisker.render()
Added whisker is a suggested dependency
Fixed error in documentation
Removed deprecation for GetlongOpt string interp

There are still some edge cases with Mustache

Keeping GetlongOpt string interpolation in for the foreseeable future
until the edge cases with Mustache are worked out

Example edge cases:

Mustache still won't access slots: myObject@var

Mustache syntax does not handle function calls very well.
Will need to write specific examples
and explore more what kind of function calls are possible
and what are not.