Allow multiple go-feature-flag

[thomaspoignant]

Description

This PR allows you to have multiple go-feature-flag instances in the same application.
Nothing changes if you don't need it, you can still use the built-in singleton like provided by the package.

If you need to use files from different places you can create your own instance of go-feature-flag:

x, err := ffclient.New(Config{ Retriever: &ffclient.HTTPRetriever{{URL: "http://example.com/test.yaml",}})
defer x.Close()
y, err := ffclient.New(Config{ Retriever: &ffclient.HTTPRetriever{{URL: "http://example.com/test2.yaml",}})
defer y.Close()

user := ffuser.NewUser("user-key")
x.BoolVariation("test-flag", user, false)
y.BoolVariation("test-flag", user, false)
// ...

Changes include

• Bugfix (non-breaking change that solves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking changes (change that is not backward-comptible and/or changes current functionality)

Closes issue(s)

Resolve #56

Checklist

• I have tested this code
• I have added unit test to cover this code
• I have updated the Readme
• I have followed the contributing guide

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