Add command line argument to run linter only on changed files

./tests/tools/check_source.sh --only-changes

It's recommended to run check_source.sh automatically on each commit,
so the rules are always enforced.
Even better is to use docker for that, so the versions of pep8 and
pylint are the same as on pipeline server.

To do this couple of lines should be added into repository's hgrc file.

[----------- cut from .hg/hgrc------------------]

[hooks]
precommit.linter = ./tests/tools/check_source.sh --only-changes
# precommit.linter = docker run -it --volume=$PWD:/beremiz --workdir="/beremiz" --volume=$PWD/../CanFestival-3:/CanFestival-3 --memory=1g --entrypoint=/beremiz/tests/tools/check_source.sh skvorl/beremiz-requirements --only-changes

[-----------------------------------------------]

These classes are intended for use server-side.

e.g. in a django view.py :

    from pyjs.jsonrpc.django import JSONService, jsonremote

    jsonservice = JSONRPCService()

    @jsonremote(jsonservice)
    def test(request, echo_param):
         return "echoing the param back: %s" % echo_param