Add conf setting for supported_nips array

[ismyhc]

Id like to limit a bunch of kinds by using the write plugin, however id also like to reflect these policies in the supported_nips array that's returned by the relay info JSON payload.

For example say I don't want to allow kind 3 (nip2)

Id like to be able to remove nip 2 from the supported_nips array. I may also want to add a nip to that array.

Right now I assume I would just from the repo and change the values in code, but having a conf setting would be ideal.

[hoytech]

I just pushed this feature to master branch: 08f0633