Changes:
* Bug fix - bat adds markers only when a file path is passed and it can use git2
on it. It doesn't add markers when bytes are passed. Hence, the code is
adjusted accordingly. The sideeffect is files being opened multiple
times and its content being unnecessarily loaded in memory.
* Refactoring of the crate - Config is extracted to its struct file.
Repetitive blocks of code are dried and nested conditionals are
flattened.
