Added the always_run option in excepthook integration

[tolgahanuzun]

import sentry_sdk
from sentry_sdk.integrations.excepthook import ExcepthookIntegration

sentry_sdk.init(
    'https...',
    integrations=[ExcepthookIntegration(always_run=True)]
)

Fix #360

[untitaker]

lgtm, could you add a test for this? Also linters are failing (see CONTRIBUTING.md for running those)

[untitaker]

Thanks! Do you want to add docs here? https://github.com/getsentry/sentry-docs/blob/master/src/collections/_documentation/platforms/python/default-integrations.md#excepthook Ideally with a code example.