Support whitespace around variable names

[jabley]

With reference to the mustache spec at github.com/mustache/spec, the
current definition for tag content:

The tag's content MUST be a non-whitespace character sequence NOT
containing the current closing delimiter.

However, later tests state “Superfluous in-tag whitespace should be
ignored.” and include tests for {{ string }} and {{{ string }}}.

That would tend to suggest that whitespace is allowed between the
opening and closing delimiters, and the variable name. That’s how other
implementations seem to work.