More than {{ is acceptable to start a tag, which means that `{{{Foo}}` won't be interpreted as "the content of `Foo`"
and should be rejected. For the sake of clarity and parsing, I suspect that those symbol should be rejected elsewhere
too.
Similary `{{Foo}}}` won't be interpreted as "Show the content of field `Foo}`" even if this field exists, so it's better
to reject `}`. It's clearly necessary to reject "}}" inside the field name, rejecting "}" seems easier to explain and
avoid future unexpected problem if the templates change.
The ":" are used to separate filters, and rejecting it in field name would ensure that there is no ambiguity.
