Allow to configure default gitignore's + header separation

[skwde]

It would be nice

• if one could configure default gitignore's to add when creating one.
• have a some kind of seperator comment string to distinguish between the different section.

Specifying something like

{
    gitignore.defaults = ['Global/Windows.gitignore', 'Global/Linux.gitignore', 'Global/MacOS.gitignore']
}

in settings.json should add those to the gitignore by default.

The header for the MacOS section could look something like

###############################################
# ## Global/MacOS.gitignore