Log-file-name access to check for CLI argument presence

[martin-raden]

This is a

• [ ] Breaking change
• [x] New feature
• [ ] Bugfix

I have

• [x] Merged in the latest upstream changes
• [x] Updated CHANGELOG.md
• [x] Updated README.md
• [ ] Run the tests

Making the variable el::base::consts::kDefaultLogFileParam publicly available via the header enables dedicated logging setups if the user specified the --default-log-file command line argument, which is eg needed within my application. I have added respective documentation to the README.md.

Thanks for consideration! best, Martin