Only generate a log file if specifically requested, in test mode or on a failure

Currently when running `ie` the log file `ie.log` is generated always irrespective of end result. This works great while user is adding support for `ie` in a new document but not that useful if we know the execution will be successful. Perhaps it would make sense to only write `ie.log` in case the execution fails, or user explicitly requests by a flag like `--log-file`.

```
→ ie execute tutorial.md
Welcome to the innovation Engine Tutorial
1. Running simple bash commands

✔ export VAR="Hello World"

✔ echo $VAR

Hello World

2. Test Code block with expected output

✔ echo "Hello \
world"

Hello world

3. Test Code block matches expected regex

✔ echo "Foo Bar"

Foo Bar

→ ls -la | grep ie.log
-rw-rw-r-- 1 qasim qasim 2331 Dez 6 16:16 ie.log
```