#2 new directive 'erepace' and 'creplace'

[Kuwata Makoto]

New directive 'ereplace' replaces an element, and
'creplace' replaces a content.
Argument may be '@element(name)', '@content(name)', or
expression.

ex.
<div id="ereplace:@element(foo)">...</div>
=> replaced the element by element named 'foo'. (This
is equal to id="replace:name")

<div id="ereplace:@content(foo)">...</div>
=> replaced the element by content of element named 'foo'.

<div id="ereplace:var1">...</div>
=> replaced the element by value of expression 'var1'.

<div id="creplace:@element(foo)">...</div>
=> replaced the content by element named 'foo'. (This
is equal to id="placeholder:name")

<div id="creplace:@content(foo)">...</div>
=> replaced the content by content of the element named
'foo'.

<div id="creplace:var1">...</div>
=> replaced the content by value of expression 'var1'.
(This is equal to id="value:var1")