Implement handling of `CallExpr`

[kornicameister]

Taken from: #49

Code examples:

    logger.debug(f'This is a test')           # 1
    logger.trace(f'Call from {__file__}')   # 2

Expected result: If loguru-mypy detects that it is dealing with CallExpr we should try and figure a final value, i.e. after applying f-string machinery.