overwrite default Options from SuperFlag string

[karlmcguire]

(This relates to DGRAPH-3003 and is blocked by dgraph-io/ristretto#247.)

1. I don't think there's a good way to include slices in a SuperFlag, so in overwriteOptions I only look at scalar, non-interface fields. I figured people could just use WithLogger and WithEncryptionKey instead of trying to accommodate it in SuperFlag.
2. How should we handle Dir and ValueDir which previously didn't really have a default value?

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This change is 

[manishrjain]

Got a comment about adding a test with a legit value for super flag.

Reviewed 4 of 4 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @jarifibrahim and @karlmcguire)

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options_test.go, line 12 at r1 (raw file):

func TestOptions(t *testing.T) {
	// copy all the default options over to a big SuperFlag string
	defaultSuperFlag := generateSuperFlag(DefaultOptions(""))

would be good to try to overwrite some option, to capture the most common use case.

[manishrjain]

@aman-bansal -- Can you resolve this and get it merged today?

Reviewable status: 2 of 4 files reviewed, 3 unresolved discussions (waiting on @jarifibrahim, @karlmcguire, and @manishrjain)

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options.go, line 192 at r2 (raw file):

//
// Unsupported: Options.Logger, Options.EncryptionKey
func SetSuperFlag(superflag string, options Options) Options {

(Options) WithSuperFlag(superflag string) Options {
 ...
}

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options_test.go, line 1 at r2 (raw file):

package badger

license here.