Allow users to retrieve JWT from a query and cookie with a specified name

[JunNishimura]

Fixes #88

1. I defined the function TokenFromCookieWith to retrieve jwt from a cookie with a name passed as an argument.
2. I defined the function TokenFromQueryWith to retrieve jwt from a query with a name passed as an argument.

How to Use

func Verifier(ja *JWTAuth) func(http.Handler) http.Handler {
	return func(next http.Handler) http.Handler {
		return Verify(ja, TokenFromQueryWith("custom-param"), TokenFromHeader, TokenFromCookieWith("custom-cookie"))(next)
	}
}

[VojtechVitek]

LGTM.

However, could you think of a better name for the new functions? To me, With() implies some with context pkg.

I'm thinking jwtauth.TokenFromCookieByName("session") or jwtauth.GetTokenFromCookie("session").

Thoughts?

[VojtechVitek]

btw: there's a third function that's similar -- TokenFromHeader().

[VojtechVitek]

a single const (for cookie/header/query param) would be enough

[JunNishimura]

@VojtechVitek

Thanks for the review, and sorry for the delayed response.

I changed the name of functions and cleaned up constants. It would be thankful if you check them again!

Thanks!