Make the casbin middleware enforcer config customizable

[rolandjitsu]

Use the casbin.IEnforcer interface to allow different enforcers to be passed to the casbin middleware config.

So now we can do:

ce, _ := casbin.NewSyncedEnforcer("auth_model.conf", "auth_policy.csv")
h := Middleware(ce)(func(c echo.Context) error {
	return c.String(http.StatusOK, "test")
})

[codecov]

Codecov Report

Merging #58 (d4923d3) into master (b7e53bc) will increase coverage by 0.28%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
+ Coverage   55.77%   56.06%   +0.28%     
==========================================
  Files           8        8              
  Lines         459      462       +3     
==========================================
+ Hits          256      259       +3     
  Misses        183      183              
  Partials       20       20              

Impacted Files	Coverage Δ
casbin/casbin.go	86.20% <100.00%> (+1.59%)	⬆️

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[aldas]

Closing in favor of #66