allure-robotframework creates separate attachment files for log messages from each keyword

#### I'm submitting a ...
- [***] bug-feature report

#### What is the current behavior?
Currently allure-robotframework creates separate attachment files for log messages from each keyword, no matter if it is from external library or from builtin Log keyword.

#### If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem
Unfortunately can't provide any test results. I think, that it will be enough to create few tests with Log keyword to see result.

To launch test project i use this command line:
robot --listener allure_robotframework --loglevel DEBUG -T --outputdir .\log\robot --variable REMOTE_URI:127.0.0.1:1234 --console verbose -s 'List_Test' .\x-tests

#### What is the expected behavior?
I expect that all **logging** attachments for whole test project run should go in to one attachment file.

#### What is the motivation / use case for changing the behavior?
This behavior can inflict problem with accessing output folder when running complex test suites with many log messages. For example, simple test suite with two normal tests generates 84 attachment files. You may imagine what will happens with output directory when a really big test project will run few times.

#### Please tell us about your environment:

- Allure version: 2.10.0
- Test framework: Robot Framework 3.1 (Python 3.6.7 on win32)
- Allure adaptor: allure-robotframework@2.6.0

Inizia riproducendo il problema con la keyword Log di Robot Framework e il comando fornito robot --listener allure_robotframework, confrontando il numero di allegati per una suite di piccole dimensioni. Traccia la gestione dei messaggi di log del listener allure_robotframework. Il lavoro è completato quando gli allegati di logging dell’intera esecuzione vengono consolidati in un unico file senza perdere messaggi, con il comportamento verificato su una suite più grande.

Scritto dal modello di indicizzazione a partire dal testo della issue.