Add disable_block_exclusion Option

[luskjh]

Use case: I want coveragepy report the coverage of lines executed after Django has bootstrapped my application - this allows more accurate coverage reporting of things like class-based views, which always show 100% when the Django bootstrap process is included in the reporting. To do so, I've created a custom test runner in my project:

class CodeCoverageRunner(DiscoverRunner):
    ...

    def run_tests(self, test_labels, **kwargs):
        self._cov.start()

        super().run_tests(test_labels, **kwargs)

        self._cov.stop()
        self._cov.save()
        self._cov.report()

this works as expected, but (also, as expected), class / function definiton lines are not reported as being run, so when I exclude them (as well as import statements):

exclude_lines = [
    "import.*",
    "from.*import.*",
    "@.*",
    "def.*\\(.*\\).*:",
    "class.*\\(.*\\).*:",
]

I run into the problem that their body defintions also get excluded per current functionality. I'v added the disable_block_exclusion option to the [report] sectiont to allow for this functionality to be disabled.

First time contributing, read through Contributing to coverage.py to try and include everything necessary here.

Open to any feedbabck from the configuration value's name to why you may not want to accept this change into your project - I would find it very useful though!