Add optional patterns

[jgrund]

As far as I can tell there is not a way to have an optional pattern.

This would be nice for matching pragmas that may not be in every file.

[silvenon]

I would classify this as an ESLint issue. Sometimes you want some ESLint rules applied to some files, but not others, it doesn't only apply to eslint-plugin-header.

[silvenon]

Never mind, I think I misread the issue.

[iocalebs]

Here's an example where this would be useful.

On this project, we need the licence header to be present at all times, but the //@flow directive is optional.

(In this case one's a block comment and one's a line comment, but that's another issue.)

[nbfowler]

Did this ever happen? I would really like to be able to make some header lines optional but I don't see a way to do that.