Support ~ for eliminating whitespaces between nodes

[sunng87]

Syntax

{{#each nav ~}}
  <a href="{{url}}">
    {{~#if test}}
      {{~title}}
    {{~^~}}
      Empty
    {{~/if~}}
  </a>
{{~/each}}

~ is to remove whitespaces before or after helpers

[sunng87]

The current parser should have issue when dealing with {{{~ xxx ~}}}. The current workaround is to write in this way: {{~{ xxx }~}}.