Implement handling of `CallExpr`

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.
