Rename .custom.gitignore

[bilaalrashid]

The name .custom.gitignore is difficult to remember, and I often find myself forgetting it. We could also take this opportunity to replace custom with a more meaningful word.

A form of .gitignore.<$name> would be more appropriate. Some options could be:

• .gitignore.local
• .gitignore.overrides
• .gitignore.custom

We will also want to make sure that we ensure backwards compatibility with .custom.gitignore whatever option we choose.

Alternative options here could be to allow all three options listed above, or to allow the user to specific the name of the file using a flag.