Add method that allows user to specify DataDir directly

[Luoxin]

Add a method that allows users to directly specify DataDir, simplifying the call path when users only want to change the DataDir value

[Luoxin]

#98

[kelvinmwinuka]

@Luoxin

What's the benefit of this change over using WithConfig in the following manner?

config := echovault.DefaultConfig()
config.DataDir = "./data"
cache, err := echovault.NewEchoVault(echovault.WithConfig(config))
if err != nil {
  log.Fatal(err)
}

I think it's an interesting idea to provide an option like this for editing config. However, for consistency, we would have to provide this for all other configurable values as well, as users will expect them.

If you'd like to go this route, I'd recommend you create these options for all the config options and add them to the PR. We should also move all these options to the echovault/config.go file.

[Luoxin]

The user can complete the operation of the NewEChovault instead of adding a lot of lines directly through the line of Withxxx

I will make some modifications according to your suggestions.

[Luoxin]

I completed the method according to the existing fields in config.Config, with special handling for bool type data, which is considered true when no bool type value is passed in.

[kelvinmwinuka]

Looks good!