Add CHANGEATTRS and htmlAttrPair

Mistuke has pointed out that (!) does not allow attributes to be added
to a general (X)HTML tag while taking account of the attributes that
are already defined by the tag [1]. This can make it hard to generally
extend tags with attributes while even being sure that correct (X)HTML
is being generated (the standard prohibits the duplication of
attributes[2]).

In order to minimize disruption the existing interface has been
extended with an alternative class to `ADDATTRS` called `CHANGEATTRS`
and a deconstructor function, `htmlAttrPair`, for analysing the
(abstract) `HtmlAttr` type. With `CHANGEATTRS` a function is used to
transform the existing attributes (which can now be analysed with
`htmlAttrPair`) rather than being passed a list of attributes to add
to an HTML tag as is the case with `ADDATTRS`.

[1] https://github.com/haskell/xhtml/issues/2

[2] http://www.w3.org/WAI/GL/WCAG20-TECHS/H94.html