adding px option for SET command

[SyedMa3]

• Added support for the PX flag in the SET command.
• Added unit and integration tests.
• Refactored ParseCommand to reuse it in unit tests.

Fixes: Issue #200

[arpitbbhayani]

@SyedMa3 There are some merge conflicts. Can you resolve them, I did skim your changes once. But will deep dive better after the conflicts are resolved.

[soumya-codes]

LGTM. cc @arpitbbhayani @JyotinderSingh do we need to add corresponding JSON support as part of this PR or do we have a separate PR for this?

@SyedMa3 Just one minor change for better readability. You may do a:

arg := strings.ToUpper(args[i])

This makes your code a little more readable: The related conditions are now reduced to:

case "EX", "PX":

and if arg == "EX" {

[arpitbbhayani]

There were some conflicts. I resolved them and merged the PR now.

Thanks @SyedMa3 and @soumya-codes!