Allow specifying startup options for Shell/Terminal

[goran-w]

In Preferences - Integration - Shell/Terminal, we should be able to specify startup options:

• I'm using Git Bash (from Git for Windows) and find it rather frustrating that it does not open (from within SourceGit) with the same options (mainly --login) as when using the (default-provided) "Open Git Bash here" in Windows Explorer. (This means that when opened from SourceGit, Git Bash does NOT run any commands from my Bash-profile config files...)
• I tried adding the missing option --login in the "Path" textbox, but it doesn't work. Besides, this causes a misleading error message "Terminal is not specified!" (while in fact I HAD specified a terminal - it just failed to open due to the extra options being added to the executable path itself).
• Adding another textbox "Startup options" (or similar) would allow me to add the missing flag --login, making Git Bash open as expected.
• I imagine this textbox would be very useful for customizing any of the available terminal variants (Git Bash, PowerShell, Command Prompt, Windows Terminal, etc - on any platform as well).

[goran-w]

@love-linger - Thanks, but while the recent commit works (after letting SourceGit auto-detect Git Bash anew) for replicating the exact behavior of the default-provided "Open Git Bash Here", it does nothing to improve customization abilities more broadly.

Adding support for custom shell/Terminal startup options should be an easy fix (I think) and would be much more useful (across all the selectable terminals).

[love-linger]

Done.

[goran-w]

Perfect, thanks!