I copied Jinja2 syntax for my PoC, but Jinja2 and D both use curly
braces a lot, making templates hard to read when they're mixed together.

The new syntax comes from balancing the following criteria:

* Easy to read when mixed with D code
* Easy to remember
* Easy to type for common cases
* Rare syntax collisions with common use cases (i.e, templating D,
  YAML/JSON, XML-like languages, etc)

Some collisions are bound to happen.  [< can be confused with common CDATA
usage.  [: can be confused with IPv6 addresses and a common regex character
class syntax not used in D.  `raw` and `endraw` will need to be used in
these cases.  I hope there aren't any seriously annoying cases.