- Added compiler support for a whitespace control mechanism. For any tag
in spacebars, using a ~ on either end of the tag will trim the
whitespace on that end of the tag. For example:

  - {{~ #if }} will trim leading whitespace
  - {{ #if ~}} will trim trailing whitespace
  - {{~ #if ~}} will trim leading and trailing whitespace

- The syntax is supported fully with extensive test cases.
- A ws_ctl flag is added to each tag for use in rendering.