Use HTMLBars plugin system to transform non-standard templates.

* Enables removal of custom `preprocessArguments` code for `{{each}}` and `{{with}}`.
* Paves the way for a more automatic streamification of arguments.
* Dramatically simplifies `each`/`with` helper code.

Requires HTMLBars v0.1.12 (changelog below):

* Robust tracking of quoted and unquoted attributes.
* Refactor AST to always treat sexprs as an array.
* Add compiler plugins.

Compare view:

https://github.com/tildeio/htmlbars/compare/v0.1.11...v0.1.12

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Credit to @mmun for most of this code, any mistakes are my own.