Make generate_jwt/2 available to sign payload with any key

[nelsonic]

At present we only have generate_jwt!/1 which takes a claims Map but uses a default signing key:

auth_plug/lib/token.ex

Lines 21 to 27 in 5936433

	def generate_jwt!(claims) do
	{:ok, token, _claims} =
	token_config()
	|> Joken.generate_and_sign(claims, @signer)
	token
	end

In order to allow us to sign JWTs with any key (so that each client has it's own key)
a DWYL_API_KEY dwyl/auth#42 we need the ability to supply the signing key as an argument.

Todo

Expose a the following functions that will allow us to supply any signing key:

• generate_jwt!/2
• verify_jwt/2
• verify_jwt!/2