Allow electrum servers be marked as "disabled" instead of deleting

[gcharang]

I had to delete an electrum server that went on a fork: https://github.com/KomodoPlatform/coins/pull/1464/files

Would have been better if I was able to add a key that says "disabled": true till the server is fixed

Setting "disabled": true must exclude the server from all the generated configs

cc: @smk762 @cipig

[gcharang]

in the same vein, add delisted : true for coins that needs to be delisted instead of deleting them from coins file

[gcharang]

implemented disabled for electrums in https://github.com/KomodoPlatform/coins/pull/1503