enable warnings to notify the users about the deprecated features

[aostr]

Currently when a pytest warning appears, the users only see a notification like below
================= 1 pytest-warnings, 4 error in 0.12 seconds ==================

(do not confuse this one with the python warnings mechanism).
This happens when, for example, there's a pytest hook which relies on the __multicall__.

If one wants to see what causes the warning, an additional -r w flag has to be added to the commandline, or a corresponding ini file option has to be set. Example warnings summary:

=========================== pytest-warning summary ============================ WI1 <path>/conftest.py:92 'pytest_runtest_call' hook uses deprecated __multicall__ argument ================= 1 pytest-warnings, 4 error in 0.06 seconds ==================

It was agreed that the warnings summary should be displayed by default.
What would be the way to go now?

1. We could set the -r flag to take w as a default. However, the default value is overwritten when specifying -r XYZ - or whatever else is then on the actual commandline call. The commandline flag will be taken instead of the default. Not an option, I guess, since many users already use the -r flag. The notification wouldn't simply go through.
2. Modify the body of the getreportopt() function in the _pytest.terminal and append the reportchars variable with w permanently. The question is here: how would then users silence the warnings if the wish to? Another flag, something like --set-report-chars could be introduced which would override the -r, but the plan is to limit the number of commandline options. Limiting the commandline options idea contradicts to adding a --disable-pytest-warnings flag.

Would this pull request allow overriding the appended of the w if option 2 is selected?

Or maybe you have a better idea how to enable the pytest warnings summary in another way?
Keep in mind that users should be still able to silence them.

[aostr]

I spoke with @hpk42 and I will implement a new flag --disable-pytest-warnings.
In such case, when -r w --disable-pytest-warnings is called, the warnings are silenced. In other cases even if the w is not set on the -r flag, the warnings summary is going to be displayed.