Lemoncheesecake

- **HTML report**: fix page layout for report in offline mode
- **HTML report**: make more room for check result details

- **matchers**: Add multi-line text rendering of `all_of` and `any_of` matchers description and result details
- **HTML report**: Use variable size container (tables) instead of fixed size

- **API**: fix disabled test attribute not properly taken into account in add_test_in_suite

- **CLI tools**: various fixes and improvement with disabled tests handling
- **Slack**: add a proxy option (`$SLACK_HTTP_PROXY` environment variable)
- **API**: add a new decorator function `lcc.hidden()` to hide tests/suites from the test tree

- **Slack**: better error handling when message sending fails
- **Slack**: check `$SLACK_MESSAGE_TEMPLATE` variables before starting tests
- Fix swapped session teardown outcome

- **reporting**: add new reporting backend for ReportPortal (http://reportportal.io/)
- **reporting**: add new reporting backend for Slack
- **API**: `lcc.prepare_attachment` is now a context manager that returns the attachment file path