Add HUGOFLAGS env var

[bep]

Saw this in the Go CLI, which I would find useful having something similar for Hugo:

GOFLAGS=-mod=vendor

E.g.

HUGOFLAGS="-D --path-warnings"

Or something.

[Boot-Error]

Hello OP,
This sounds super useful! I'm willing to take up this issue.
Please hint me some pointers in the codebase to starts hacking 😃

Here

hugo/main.go

Line 23 in 0e75af7

resp := commands.Execute(os.Args[1:])

the value of HUGOFLAGS can be added along with os.Args[1:].