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We intentionally try to enlarge the configuration space only as little
as needed, thus we partition the space for the cabal solver into

 - xhtml-3000.2.2   supports base  < 4.11 & cabal-version: 1.10
 - xhtml-3000.2.2.1 supports base >= 4.11 & cabal-version: 2.0

as xhtml-3000.2.2 and xhtml-3000.2.2.1 are not distinguishable except
for their version number.

This also bumps xhtml's version to 3000.2.2, since the new instance
represents a user-visible API addition.

Add .travis.yml and convert README to markdown

And add a travis build status badge.

Fix compiler warnings

Tested with ghc 7.8.4, 7.10.3 and 8.0.1.

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

Update .gitignore

Use https url to git repository in cabal file

This way it gets hyperlinked on Hackage, and still works with `git
clone`.