I think you would implement this by using a scripting language like python or nodejs that simply calls the api with a http get call every x minutes (using cron) and updates your client via rpc. With nodejs this would be something like this (untested coffescript):
rpc = require("node-json-rpc")
CronJob = require('cron').CronJob
http = require("http")
options = #place bitshares rpc settings here
port: 5080
host: "127.0.0.1"
client = new rpc.Client(options)
c = new CronJob "'00 30 * * * *'", -> #run this hourly
http.get "http://coinmarketcap-nexuist.rhcloud.com/api/btsx/price", (price) ->
client.call
method: "wallet_publish_price_feed"
params: [
"yourdelegatename"
price.usd
]
id: 0
, (err, res) ->
throw err if err
c.start()