widen the dependency on 'package:csslib' (#217)

* widen the dependency on 'package:csslib'

* update CI

* Update CHANGELOG.md

added package topics to the pubspec file (#215)

* added package topics to the pubspec file

* Update pubspec.yaml

prep for publishing 0.15.2 (#205)

rev the package version in preparation for publishing (#190)

Prepare to publish (#152)

Fix infinite loop with bad button close tag (#128)

Closes #122

Add 'button' to the tags that are processed with `endTagBlock`. The spec
at https://html.spec.whatwg.org/multipage/parsing.html includes 'button'
in the list of end tags with this treatment.

Add a new `.dat` file for regression test. It include an `#info` field
which is new but can be parsed safely to mark the issue for which it is
a test. This test loops endlessly before the fix and passes after the
fix.

Note that since errors are currently untested and tests fail if they are
tested the error will be left without content for now.

Bump version in pubspec in preparation to publish (#110)