Add option for custom formatter

[markus-work]

Hi,

I have expanded the pytest options with --custom-formatter-path where you can input a module path to a file containing the class CustomFormatter.

[liiight]

wow, I can't believe people use this, not to mention actually put up a PR! Thanks for this!
I wonder what is your use case for this?

[markus-work]

My tests don't have classes, so I got lines with only ## and I did not want that. I could also remove the markers, since I am not interested in that. And since I had implemented it, I might as well share it :)

In my formatter I also handle multiline docstrings. Here it is:

from pytest_docs.formatter import Formatter
from textwrap import dedent


class CustomFormatter(Formatter):
    name = "custom"
    marker_prefix = ""

    @staticmethod
    def module_name_format(element):
        return "# {}".format(element)

    @staticmethod
    def class_name_format(element):
        if str(element) == "":
            return ""
        return "## Class {}".format(element)

    @staticmethod
    def func_name_format(element):
        return "## {}".format(element)

    @staticmethod
    def marker_format(marker):
        return ""

    @staticmethod
    def func_desc_format(element):
        return dedent(str(element)).lstrip("\n")

[liiight]

hmm, I'm a bit hesitant to introduce such a direct import mechanism system to support this use type. I think refactoring the existing formatter to be plugins and loaded as such would be better, as it will allow any user just creating new ones on their end without needing new code changes.