Ktlint should apply the dogfooding principal and only provide experimental
rules that at least on the ktlint code base itself gives satisfiable
results.

Initially all experimental rules that cause violations have been disabled,
so that a separate commit can be created to enable each specific rule.

For BaselineTests it was necessary to rename the files which are used for testing
to have a non standard kotlin file extension. This prevents the files from being
changed when running the ktlint formatting on the ktlint code base itself. Note
that the baseline protection mechanism did work in this case and as of that has
been removed from the command.

It is not yet possible to enable the rule as some violations are actually false
positives. This will be solved by pinterest#1284

For now, it has been chosen to disallow trailing comma's instead of forcing them to
be added. Reasons for this are two folded. The number of changes is considerably
smaller. More importantly is that the benefit, with respect to avoiding future merge
conflicts, seems not that big when scanning the code change which would result from
forcing the trailing comma to be added.

There is no need for a separate build task to run the experimental rules. The
experimental rules can be executed by default in the "ktlint" task. Also, the
baseline has been fixed so there is no longer a need to use extension "_kt"
for the baseline test files.

Closes pinterest#1222

…perimental-rules

# Conflicts:
#	CHANGELOG.md
#	build.gradle

Those source files all contain linting errors which have to be reported
by unit tests. Therefore they should not be reported during a normal
build because those should not be fixed as that would result in the
tests to fail.