If given a string, it sets the splice argument name to it. It
returns the old splice argument name.
If a splice argument name is set, it's parsed in all tags, both
those with callbacks and those without. Wherever it occurs, its
value (after being parsed for entities in the normal way) is
inserted directly into the tag. E.g:
<foo arg1="val 1" splice="arg2='val 2' arg3" arg4>
becomes
<foo arg1="val 1" arg2='val 2' arg3 arg4>
if "splice"
is set as the splice argument name.