Add support for subexpressions.

    {{foo (bar 3)}}

Mimics functionality of handlebars.js. Subexpressions are always
evaluated as helpers; if `3` were omitted from the above example,
`var` would be invoked as a param-less elper, even though a
top-level `{{bar}}` would be considered ambiguous.
Merge pull request #75 from ebernhardson/subexpressions

Add support for subexpressions.