feat: add option to define flag and permissions of the file

[ldez]

To avoid breaking changes (constructor signature and existing behavior), I extended the current constructor with a variadic of functions to add options.

The approach avoids creating a new constructor, like NewWithOptions(path string, opts Options), and breaking the current constructor signature.

func New(path string, opts ...Option) *Flock {
    ...
}

There are 2 options:

• SetFlag: sets the flag used to create/open the file.
• SetPermissions: sets the OS permissions to set on the file.

    _ = flock.New("example", flock.SetFlag(os.O_CREATE|os.O_RDWR)))
    _ = flock.New("example", flock.SetPermissions(os.FileMode(0o600)))
    _ = flock.New("example", flock.SetFlag(os.O_CREATE|os.O_RDWR), flock.SetPermissions(os.FileMode(0o600)))

Fixes #59