restructure pattern matches to avoid annoying compiler warnings about
analysis budget

the suggested workaround would require a java option and is therefore
not easy to fix for everyone
align project description in build with GitHub description
simplify pattern match in sbt plugin
avoid scaladoc warnings about undefined variables in code sample
add target directory of Laika build to cleanFiles in sbt
avoid non-termination when parsing nested spans in the block directive
parser
dropping support for 2.9 allows the use of implicit classes now

they cannot be Value Classes in this case as they do not meet the
requirements