Provide means to trim return from <:sub ...> </:>

[jonathanstowe]

I've just been caught out by the return of a <:sub> including the \n at the end of the line when formatted like:

<:sub form-name($target)>
   resyncForm_<$target.dc-id>_<$target.channel-id>
</:>

Obviously if one is using that as an attribute value (as I was,) then that won't have the outcome desired.

Clearly one could format the sub as:

<:sub form-name($target)>resyncForm_<$target.dc-id>_<$target.channel-id></:>

To lose the \n but that's less than lovely.

Perhaps there could an additional attribute to the <:sub> (and the similar things,) which could cause the leading and trailing whitespace to be trimmed from the return. It would be nicer if this were in the template itself rather than in the calling code as it may want to be on a case by case basis.