Proof of Freshness. Run with:

	$ go get github.com/esote/pof && pof

Collate values from multiple sources. Used to prove something cannot have been
prepared ahead of the date specified within a day or so.

Current sources:
	- arXiv recently submitted preprints
	- Cryptocurrency block hashes (depth 10)
		- Bitcoin
		- Monero
	- News (5 latest articles)
		- Der Spiegel international
		- New York Times world news
		- BBC world news
		- The Economist latest updates
	- NIST beacon v2

Inspired by the proof of freshness used in Qubes OS's canaries.

Example output:

	Date: 2020-07-27 02:57 UTC

	Src: arXiv (https://export.arxiv.org/api/query?search_query=all&sortBy=submittedDate&sortOrder=descending&max_results=10)
	 ---
	Time-travelling billiard ball clocks: a quantum model (Lachlan Bishop, et al., 2020-07-24T17:45:35Z)
	The Relativistic Jet Dichotomy and the End of the Blazar Sequence (Mary Keenan, et al., 2020-07-24T17:27:28Z)
	McKean-Vlasov SDEs in nonlinear filtering (Sahani Pathiraja, et al., 2020-07-24T17:22:34Z)
	Obstructions to matricial stability of discrete groups and almost flat K-theory (Marius Dadarlat, 2020-07-24T17:12:16Z)
	Beating Greedy For Approximating Reserve Prices in Multi-Unit VCG Auctions (Mahsa Derakhshan, et al., 2020-07-24T17:08:42Z)
	Linear Symmetries of the Unsquared Measurement Variety (Ioannis Gkioulekas, et al., 2020-07-24T16:59:40Z)
	Magnus Nernst and thermal Hall effect (Debottam Mandal, et al., 2020-07-24T16:55:00Z)
	All-optical density downramp injection in electron-driven plasma wakefield accel... (D. Ullmann, et al., 2020-07-24T16:34:57Z)
	Corpse Reviver: Sound and Efficient Gradual Typing via Contract Verification (Cameron Moy, et al., 2020-07-24T16:30:42Z)
	Bayesian Combinatorial Multi-Study Factor Analysis (Isabella N. Grabski, et al., 2020-07-24T16:17:47Z)

	Src: Blockchain.Info [depth 10] (https://blockchain.info/block-height/640964?format=json)
	 ---
	0000000000000000000ea75860ec92c515a71584ac6f279e4a4ce995625e91c2

	Src: Moneroblocks.Info [depth 10] (https://moneroblocks.info/api/get_block_header/2151095)
	 ---
	e9c7c3f3a8987dc4a386f335c3ed61f8aaf177b06d3e827affd214c7fc6b45dd

	Src: Der Spiegel international (https://www.spiegel.de/international/index.rss)
	 ---
	Trump s Handling of the Coronavirus Could Cost Him Presidency
	How Syrians Are Reshaping German Society
	Cheap and Expendible: The Systematic Exploitation of Harvest Workers in Europe
	Muslim Calls to Prayers Get a Mixed Reception in Germany
	VW, BWM, Daimler: The Race to Beat Tesla at Its Own Game

	Src: New York Times world news (https://rss.nytimes.com/services/xml/rss/nyt/World.xml)
	 ---
	Officials Push U.S.-China Relations Toward Point of No Return
	As the World Gets Tougher on China, Japan Tries to Thread a Needle
	Protests Swell in Russia s Far East in a Stark New Challenge to Putin
	For French-Algerian Families, Virus Disrupts Cherished Summer Ritual
	Brazil s Bolsonaro, Leading Virus Skeptic, Says He s No Longer Infected

	Src: BBC world news (https://feeds.bbci.co.uk/news/world/rss.xml)
	 ---
	US diplomats to leave consulate in Chengdu in tit-for-tat closures
	Coronavirus: Spain says outbreaks under control after UK orders quarantine
	Olivia de Havilland, Golden Age of Hollywood star, dies at 104
	The gravedigger s truth: Hidden coronavirus deaths
	John Lewis: US civil rights icon's body crosses Selma bridge a final time

	Src: The Economist latest updates (https://www.economist.com/latest/rss.xml)
	 ---
	Donald Trump cancels his Florida celebration
	The world s leading brands jump on the direct-selling bandwagon
	The Greece of dreams and nightmares
	Checks and Balance our weekly podcast on American politics
	Is 2020 turning into a  law and order  election?

	Src: NIST beacon v2 (https://beacon.nist.gov/beacon/2.0/pulse/last)
	 ---
	C12C220AEE18C9ED3991EB5D513BEBC5CEF2C62BD22ADCA8737BEF6B5E195BA87D17CC3DC39D4C38B3D04EF477687E50F82DA8CB11A84443AFA77E940B10364B