If given a string, it sets the splice argument name to it. It
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.