Allow customize Shell & ShellArg in config.yml

**Is your feature request related to a problem? Please describe.**
I have tons of aliases but cannot use them directly in Lazygit because bash is the default shell.
I use zsh.
I found some solutions like `zsh -ic`, but I need to prefix it every time, it is so inconvenient.

**Describe the solution you'd like**
Support customization in config.yml like:
```
os:
shell: "",
shellArg: "",
```

**Describe alternatives you've considered**
Detect user shell and support alias. I found a closed PR, but didn't dig into it, maybe there are some bugs in that implementation?

**Additional context**
None