Add function for polling asynchronously

Sourced from file-icons/atom@7a70b0ce, where the function was previously
named `condition`. Renamed and simplified to be clearer for general use.

lib/functional.js

@@ -125,6 +125,30 @@ function nerf(fn, context = null){
 }
 
 
+/**
+ * Keep calling a function until it returns a truthy value.
+ *
+ * @example poll(async () => (await fetch(url)).done);
+ * @param {Function} fn
+ * @param {Object} [opts={}]
+ * @param {Number} [opts.rate=100]
+ * @param {Number} [opts.timeout=0]
+ * @param {Boolean} [opts.negate=false]
+ * @return {Promise}
+ */
+async function poll(fn, opts = {}){
+	const {rate = 100, timeout = 0, negate = false} = opts;
+	const start = Date.now();
+	for(;;){
+		const result = await fn();
+		if(!negate === !!result) return result;
+		if(timeout && Date.now() - start > timeout)
+			throw new Error("Timed out");
+		await new Promise($ => setTimeout($, rate));
+	}
+}
+
+
 /**
  * Monkey-patch an object's method with another function.
  *