Fixed handing of whitespace control characters

[idpaterson]

Comments and triple-stache expressions were not compatible with ~ whitespace control characters. This pull request adds support for the following syntaxes to allow proper code highlighting when using the control character to remove extra whitespace:

{{~! single line comment ~}}

{{~!-- multiple line 
comment --~}}

{{~{ nonEscapedExpression }~}}

The existing syntax for triple-stache non-escaped expressions was invalid ({{{~ expr ~}}}) which may be why there was no test for that specific case.