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: https://github.com/hoytech/strfry/commit/08f06338f32a91a6195973357f40b09bb2f6b076

[sondreb]

Will this limit the kinds accepted? Would like to run an instance of strfry similar to purplepages.