updated get_file_list method for folder support

[jaromieee]

To only list files from a specific folder in the target repository, you can add it to the URL like so:
`.../projects/{project_key}/repos/{repository_slug}/files/{folder}

I added an extra parameter to the method, which will add the folder to the URL if given.

[abloemert]

I see that the black command fails, but I don't fully understand why. @gonchik, can you help us out here?

black run-test: commands[0] | black --check --diff atlassian/ examples/ tests/
Traceback (most recent call last):
  File "/atlassian-python-api/.tox/black/bin/black", line 8, in <module>
    sys.exit(patched_main())
  File "/atlassian-python-api/.tox/black/lib/python3.7/site-packages/black/__init__.py", line 1372, in patched_main
    patch_click()
  File "/atlassian-python-api/.tox/black/lib/python3.7/site-packages/black/__init__.py", line 1358, in patch_click
    from click import _unicodefun
ImportError: cannot import name '_unicodefun' from 'click' (/atlassian-python-api/.tox/black/lib/python3.7/site-packages/click/__init__.py)
ERROR: InvocationError for command /atlassian-python-api/.tox/black/bin/black --check --diff atlassian/ examples/ tests/ (exited with code 1)